Just wanted say that F104 is a good car to start with, even though I do missed tweaking the F103. We just had a local F1 race consisting of a good numbers of F104s and a few F103s. It was an Open race only closed to non-rebuildable black can motor.
Our local EP star got provisional TQ on a F103 and my F104 pipped him by 0.2 secs for TQ in the last qualifying round. In the finals, His F103 won the race while I come in 2nd. 3rd to 5th were also on F104.
I guess while like what Vyger mentioned, the F103 had 15 years of development and the F104 had only a few months, I say that the F104 has great potential bearing in mind the 3rd to 5th placings F104s were new comers to the EP scene although they were veterans in GP.
I think the basic difference between these 2 are that while the F103 is more agile, the F104 is a easier car to drive out of the box.
Our local star would prob be building his F104 pretty soon!