A tough choice. I have both (came in first in one Kessel Run, and third in another). And, I am very much looking forward to getting two more TIE Interceptors so I can run a 3 TIE / 3 TIE Interceptor squad.
With the TIE Interceptor, you get 6 pilots / ship cards (including 3 unique) and 2 upgrade cards that are pretty good (daredevil and elusiveness). It is a very maneuverable ship with great attack dice given the rebels currently have 2 defense dice. Though, they seem to be priced a bit hit for me (18-27 points, compared to 12-18 for a TIE).
For the Slave 1 you get 4 pilots / ship cards (including 3 unique) and 13 upgrade cards. Many of these upgrades are very good (stealth devices X2, assault missiles, veteran instincts) and can be used on other ships, and a few others can only be used with the Slave 1 but add a lot of fun/variety to the game -- though I’m not sure how competitive they are yet (seismic charges and proximity mines). But, I find this ship much harder to move given the large base size (you often run into things, or they run into you).
What makes this especially hard is that in the end I think I will use the 3 TIE Interceptors more than a Slave 1, but that I will use the Slave 1 upgrades more than the TIE Interceptor’s. I’m sure there are other pros and cons to each, too.
The good news is that I don’t think you can make a bad choice. If you want more ships with modest upgrades, go with the 2 TIE Interceptors. If you don’t mind getting one ship with lots of great upgrades that can be used on your current ships (that also happens to be a fun ship to play from time to time), go with the Slave 1.