Durability and speed, the two are rather alike. Cronos is a multiplier, it, like Icculus said, gives any unit within its range their PT, it isn't really meant to be killy, though a well placed unscattered blast template and good dice rolls, it could really put a dent in some
MEQ units, biggest problem is that it has an even worse range than Talos, and once it's done it's PT dispensary job, it really can't do much else except to hold objective in that one mission. Talos, on the other hand, is a slaughter machine, providing you can get it up close, it doesn't provide any support for the rest of the army except killing things, and being attention magnet, though Cronos can do that 2nd part as well, if you need
AT, give it
TL-HWB and you can reliably drop a vehicle's
HP by 1 a turn, or
TL-
HL, though I'd say that's better for if you're WWPing, it just takes too long to get into that melta range, whereas HWB is just helping from 24" on, or just stick to
TL-
SC for more AI fire.
So, it depends on what you feel your army needs. I'll be a contrarian of sort and suggest Cronos though, templates and pie templates will work fun on them nids... Horde nids. If it's
MC nids, Talos!