As a point of interest, what is your criteria if you don't mind sharing?
I just buy what I like, sometimes relevance to me personally, sometimes I just think it looks great. While I have no real affinity for for Delta, albeit having flown them quite a bit, I find I have a largish selection of Delta already given I think they do some fabulous special liveries an interesting planes.
I understand once you get into the hundreds of models you do probably need to be a bit more selective than "it just looks nice".