I would go by size and popularity;
Classic: 58mm. the first one I ever had (that many of us had)
Cadet: 84mm. Alox do I have to say more?
Spartan: 91mm. one of the more popular medium layers models (I have 3)
Swisschamp: 91mm. one of the most popular big boys with layers!
I personally like the Champion Plus because it was like 1/2 the price just without the hobby pliers
Farmer: 91mm. (actually 93mm) Alox people luv their Farmers! I have 4 (and all the colors they come in)
OneHandTrekker: 111mm. my personal fav because of the size and simple tool selection.
I got it when their were no PE versions and luv them, and I hate serrations! but Vic did them right here.
of course everyone has their preferences, what works for me may not for you but these here I have read about the most
and have the most, good luck and don't forget to tell us what you get!