Well, since I only own a Tama-Go and not a Musicstar I cannot answer for both. Less than 10 days ago I bought the tama-go, but Iv'e been familiar with the P2.
Is there anything I should know about them?
-Well, the Tama-Go screen is quite big(roughly 60x40 pixels) and has a 4-level grayscale screen, which means backgrounds are possible. Oh yeah, Tamatown is down. This means you cannot connect. It's not a drama imo for the Tama-Go, because the only thing you could do is transfer points.
Are they so terribly different than the older ones?
-Well, compared to a vintage one, yes. It has a shop where you can buy food and background. The food restocks with new items about 3 times a day and if you earned enough gotchi-points with playing games sometimes a new background can show up. They include 2(3 with the including figure) games which are a lot more advanced than a simple guessing game as the P1/P2 had. You can also visit the park and marry eventually when you're minimal 6 gotchi-years old.
Are they difficult to keep track of?
-Not really, much the same as the vintages imo. They can beep once in a while when they want attention.
Can they be paused?
-Yes, pressing A and B simultaneously causes it to pause.
Is it necessary to buy the extra figurines for the tama-go?
-Well I guess it would be extra fun, ofcourse. But it is not really that necessary to run properly or so. So far, I haven't bough any figures yet, and I'm still pleased with my tama.
Can they connect with any other versions besides their own?
-Yes, I believe so. In the connect icon, I can choose Tama-Go and other. I'm not sure however with which other versions.
Hope I could help.