Digitaldog, Katrin Eismann says to scan B&W in color mode and gives ample reasons why in her Photoshop Restoration & Retouching book.
I am starting a project to scan nearly 9000 Kodachrome 35mm slides. Maybe a dozen or 2 will later get rescanned. Won't know of such selects till the project is done and can be viewed. All are from 1977 to 2004 and shot in India. Of course all of these are color. My friend does have many historic B&W that will be scanned later.
That's why I plan to use a 5x7 general use size. I can get good letter size prints from them on my 3880 printer. Comparing a native 5x7 print to a letter size of the same file. my eyes can't see degradation.
I think it may time to do a review of fundamentals here.
Let's start with the "optical resolution of the scanner" - the maximum the hardware can resolve without resampling the data. We throw this expression around, but it doesn't necessarily mean what the scanner specs say it is. For example, you can buy a scanner that rates itself at 6000 PPI (or DPI, same thing in this context) but when you test it with a resolution target you find that the EFFECTIVE maximum resolution may top-out at 3000 (page 125 of my SilverFast book provides some sobering information on several scanners in this regard). The scanner manufacturer isn't necessarily lying - what they call 6000 may well describe how the sensor is designed, but other factors such as lens quality, the lighting system, the flatness of field, and others can impact the EFFECTIVE outcome of scanning. Those factors will not vary as a function of the resolution you select. So if the scanner can't really deliver any more than say an EFFECTIVE 3000, it's pointless packing more unusable information into the file.
Now let's look at the "storage is cheap" argument. While that may be true, it's useful to consider how much storage you'll need as a function of file size. Firstly, you'll want to back-up the files you create. So right there, double the storage you start with. Then there is processing. If you are processing them "non-destructively" in Lightroom, metadata adds very little to file size, but if you are doing so in Photoshop, layers easily double the file size, if not much more, and assuming you wish to preserve those layers (the "non-destructive" aspect) you can at least double the file size again, along with its associated back-up. So what may start life as a 60MB storage requirement, for example, can easily end-up as at least a 240MB storage requirement by the time you finish - i.e. 4x what you started with. Cheap X 4 may no longer be so cheap. Add to this the consideration that larger files can take longer to retrieve, edit and save than smaller files and you end-up perhaps consuming more time than necessary for your purposes.
9000 slides is a lot of scanning - a daunting exercise in fact. So let's look at this with a bit of arithmetic. I'm assuming these are 24 * 36mm slides, so the maximum dimension is 1.5 inches (and I'm assuming no cropping here - cropping changes the calculations). The maximum print size you EVER IN THIS LIFETIME intend to make is letter size, with say a one inch border on each side, so the maximum dimension of the photo itself is 9 inches. You want to print it in a Canon printer at a "native" resolution of the Canon driver of 300 PPI. So you need 9 x 300 pixels, or 2700 total output pixels on the maximum dimension. To pack 2700 output pixels into a 1.5 inch input, you'll need 2700/1.5 = 1800 input PPI setting for the scan resolution. Or seen in a perhaps more intuitive obverse manner, if you scan that 1.5 inches at 1800 pixels per inch, you obtain 1.5 x 1800 = 2700 total pixels, which when divided by your 9 inch output (the print) gives you the 300 PPI the printer driver likes.
Now let's see what file size that creates using the formula explained on page 124 of my book (which delivers the same results that scanner software such as SilverFast reports for the same settings): the result is 27.8 MB, assuming 16 bit depth for the scanning, and a 6*9 inch photo (say on an 8.5 x 11 inch sheet, but only the photo dimensions count). Now let's say for some reason or other you decide you want to provide for an 11 * 14 inch print (or given a 1 inch border, 9 * 12 inch printed photo on an 11* 14 sheet) even though you may never ever in your lifetime make one. Applying the same formula with all the same inputs except for the change in photo dimensions, you'll have a file size of 55.6 MB - double what you need. And, by the way, you'll need 3600 output pixels on the maximum photo dimension to achieve this result, which means you need to scan at 2400 input PPI instead of the 1800 needed for the 6*9 inch photo, so scanning will take longer.
Now that we have all that straightened away, let's look at the size of this job - as I mentioned above, daunting. You would want to automate as much as possible. For this I would recommend that you look carefully at SilverFast's batch scanning capabilities.
You will want to get the scans in reasonably good shape at the scan stage to minimize time spent in post-scan editing. How much editing to do at the scan versus the post-scan stage is a mixture of technical reality, taste and judgment, all explained in my book - too much of a long story to put in a forum post, but if I had to put the most basic advice into a nutshell for you, whatever balance you wish to strike about what edits to do where, at the least, make sure when you scan you are sending to file an image that is relatively open (no excessive contrast) without blocked-up highlights and shadows, i.e. minimizing highlight and shadow clipping, and make sure your scanner is properly colour-managed (decent profiles etc.). Once the image file is created and you need to do any further editing and printing, I would recommend using Lightroom. The edits are all meta-data, non-destructive, preserved for future editing, and the workflow from editing to printing within that one application is seamless. Lightroom also has very good tools for sharpening and noise control, which work well with moderate film grain as well.
On the colour versus B&W scanning, I think we are all agreed that if you are scanning colour originals, scan them in colour mode and if you wish to convert them to B&W, do that afterward. Lightroom is great for this. This allows you to conserve the maximum amount of control possible over the eventual tonality of the B&W renditions.