I got a 73 TX650 myself. Not a bad bike at all actually. All XS 650 series had some issues, but no more than all Honda had issues as well.
I own 3 Hondas and 2 Yamahas btw
You need to know however, that not all the 73 engine parts interchange with the later year XS' models. there are certain years that you can swap parts back and forth, but the 73 and earlier years are the harder ones to source parts for. However, that doesn't mean that you wont find anything for them. Check out Mikesxs.com for parts, 650 central, etc... you will see that there are plenty of parts made for these including the TX models. But if I was going to get an XS all over again, I would go with a 75-76 model myself. a lot of the frame issues, and engine upgrades were already sorted out by then.
I don't own any 4 cyl Hondas, But I have had 4 cylinder Yamahas. I don't know which is easier to fiddle with when it comes to the CB400F or the TX650, but 2 carbs and 2 cylinders should be easier and cheaper to mess with IMO.