I have a Sekonic 558R, which I believe operates pretty much same way as the 358. Main differences are 558R comes with a spot meter and a radio remote built into it. The radio chip can be added to the 358, as can a spot meter, but if you want both, obviously the extra $$ for the 558 is worth considering.
For metering flash the 558/358 can be set to trigger the flash via radio or sync cord, so you can set up your shot and then fire off the camera, metering the flash as either reflected or incident. They also can handle multiple flash units, and provide an average.
A cool feature they also offer, tho, is the abilty to read the flash that is triggered from the camera, or by hand, rather than triggered by the meter. So if you don't have a radio trigger on one of your flashes, for instance, and you want to take an incident reading at your subject, you can set the meter up so that when you push the "read" button on the meter, it waits until the flash and then reads it. Then you have an assistant fire off the flash and voila, you have your reading.
Or you could put the meter on a tripod next to your subject, and using the same method walk back to the camera or strobe and fire it off, and still get the reading.
These meters will also tell you the amount of ambient vs flash that makes up the reading. (I'd confirm all of the above for the 358, but I'm pretty sure it's the same - I use these on my 558 quite a bit).
In short, the Sekonics cost more money, but in my view the extra features are worth it if you're going to be doing studio/flash work with them.