Easy answer - get both. As a collector - i had to have a 2600 in my collection (although i still need to get a heavy sixer, just got the 4 switch model) The 7800 has some good games, which, for the price seemed a no brainer for me, even though i didn't have the nostalgia for it like i did other systems. I think my 7800 must be one of the later models, as iMagic carts fit just fine - i heard that on early models some 3rd party carts - particularly iMagic, either dont fit, or are extremely tight.
I honestly haven't tried all my carts in the 7800, but of those i have, every single one has worked without issue.