That bigshot will likely be an improvement over the thruster. Maybe not a huge one, but I think it would be an improvement no less. Disclaimer: I have no personal experience with either.

If I were on your budget, I'd buy the bike. Then immediately start saving for a nicer frame to put those 75's on. Look at it this way - You'd be buying one of the best track cranksets at like half of retail and just so happen to get the rest of the bike as a bonus.