Quoted from CrazyLevi:Frontier is on the average to boring side for me, it's just another "light spinner, shoot the spinner, shoot the inline drops" Bally game of which there are a million. I never really get the cult around it when there's so many other Bally games just like it that have better art and themes.
If you are going for a game with that "wow" factor - which it looks like you are into with your collection - Frontier isn't going to provide that, while Fathom certainly will. There's a reason people pay the big money for Fathom.
If you are looking cheaper how about Skateball? That's a really fun game to play, super cool art, and different than the usual Bally fare (I mean kind of. Four flippers!). It's got a nice fast layout that was stolen from Flash with some magic added.
Another option would be Stern's Ali or Stars, both great games to play with a cool factor to them.
Agreed, Fathom is a great flip-control machine, with those incredibly fun long shots up the left side to drop the top in-lines, the right in-lines, great extra ball feature, fantastic flip entrances, and the ultra cool dual bonuses, huge DT bank, man I can't see why you wouldn't want that in your line-up. Not only that, the sounds are nice and the artwork is arguably one of the best that Bally has ever produced from that period. I have one in the restoration queue following a customer's Quick Draw restoration.