Welcome.
I think the 400 would be a better bike to cafe. Mainly because the XS2 does pose a few problems and extra costs due to it being the early type, (pre74 the engines are different to the later 74-84 ones), and the XS2 has a one year only front end, (forks hub brake rotor), so parts will be harder to get if you need to replace any of these parts.
Read Mailmans excellent restore thread, he has a contents list on page one so you can go directly to parts of the restore that are of interest at the time.
http://www.xs650.com/threads/mailman’s-xs2-a-full-on-restoration.51520/
On the XS2.........Having a title, and having the bike in running condition would make it a more valuable selling prospect. Then again if the bike has been made reliable, Electrics/charging/brakes, and a clean tank with petcocks and carbs rebuilt and synced, with new rubber and a bit of a tidy up the bike can be used and ridden around. may not be a big cost and it can be sold or advertised a a running bike. You don't have to do a chrome paint restore.after it is made reliable a clean up with WD40 makes it look a million $ even in a rough state, and keeps it from deteriorating..........also has the look of a well used original bike
Unless money dictates, it might be prudent to sit on both of the bikes for a while to get a feel on what is the best way forward for yourself, after deciding what advice bes suits and doing some research on here.................cant get a better site than this one for information and hands on help.