I personally don't believe a RIP is the way to start. Current printer drivers are very good, and most paper makers create very good profiles. Ilfords new papers that are arriving very soon (which are not much different than their current papers) are all being profiled for them by xRite (according to my Mac Group rep). Almost all Epson profiles are very good. Moab also provides very good profiles.
The 3880 is a great starter machine, very reliable, rarely clogs, the only downside is the panorama thing. But it has big enough ink cartridges to make the costs more reasonable as opposed to the 3000.
The 4900 is great if you want to use roll paper, I have one, and rarely put roll paper through it. I only use it instead of a 3880 because I want an exact match to my 9900.
Visually for photography there will be no perceptible difference in prints between the two except perhaps a rare one with some extreme colors, and even then the 3880 will be very good.
Right now the 3880 has a $250 rebate, the 4900 has a $500 rebate, (in the U.S.) which means you can get a 3880 for around $1k, a 4900 for around $1600.
I agree with most of this. I don't think a RIP is necessary unless the user wants it for its layout, print management and archiving features. For print quality alone, it's unnecessary for the reasons you state.
As for paper profiles, yes, the Epson profiles are fine for Epson papers. I've not had very satifactory results with the Ilford profile for Gold Fibre Silk. I had better outcomes making and using my own profile, and even better yet letting Scott Martin make me a custom profile with the very latest X-Rite equipment and his own target. All that said, it could be that since I last tried an Ilford profile they may have improved it.
I had to laugh where you talk about your usage of roll paper in the 4900. One of the reasons I bought a 4900 was to have this capability and I too seldom use it. I was on the phone just Friday with my paper/ink supplier here in Toronto and he observed that very few of his 4900 customers use the roll holder either! Well, as is often said, the road to hell is paved with good intentions; OK not really a road to hell - remains a fine printer, but from my experience it demands more regular usage than my 3800 required to avoid the need for cleanings. There's all kinds of speculation out there about why, and I'm not a printer technologist so I don't know, but just another observation to add to the list.
When I was writing my review of the 4900 for this website http://www.luminous-landscape.com/reviews/printers/the_epson_4900_printer_hands_on_and_down_to_work.shtml
I did compare its output with that of a 3800 (one generation before the 3880) and differences of print appearance even for that comparison were "de minimus". By the way, after my review appeared, someone from Epson got in touch and commented that its better NOT to transport these machines on their sides, as shown in my first illustration! (Well, it was just a few feet).
I think the choice between a 3880 and a 4900 boils down to a few operational variables of which print quality is NOT one: need for the roll holder, price, size and weight (it needs space and a very sturdy table or cabinet) and frequency of usage.