Why are you asking on a forum how many seats you want in the cab?!?!? Surely you know how many people you'll be taking?!
A USA spec U500 has a faster top speed than Mercedes designed the older mogs to go.
A U500 has a very slightly longer wheelbase as it comes, and dzzz on this forum has proven you can extend its straight chassis rails. The U2450 does not have straight rails (and does have torque tubes), and according to folks who should know on Benzworld its a much bigger deal to extend it.
You can have a 4m long camper on either without stretching which makes for a permanent double bed/dinette/shower room camper if thats what you want, a DOKA with less room means the permanent bed will no longer really be a possibility unless you put quite a big overhang on quite a (relatively) long truck . As with all things it will end up as a compromise, will a long wheelbase and long overhang and tall camper box compromise your reasons for looking at a Mog?
Broadly speaking without a chassis stretch or big overhang the DOKA seating leaves camper sleeping for only two inside. This camper is almost an exception since with a 60% max overhang the box could be 2' longer with spare tyre elsewhere, but it looks like a squeeze!
http://www.opensens.com/unimog.htm
Personally I would really like the benefits that a U500 brings like an auto (clutch) box and vario steering, but unless I would be planning on staying close to home I think they're an unneccessary liability. As is the big curvy screen. You can rebuild both trucks to the same mechanical standard, you'll need a similar inventory of spares to take with you, you'll have similar problems if something mechanical breaks, but one stays simple the other still has bells and whistles. On a quiet dirt road broken down miles from anywhere the diagnostics on a U500 could tell me a list of possible things to look at (as far as I know anyway), but the electrical stuff surely adds another layer of stuff to break in itself?
Being in the UK means I could have a new Euro 5 mog, or any other mog and maybe a wider choice of people to work on either, but I'd still go old