The two computers are aimed at different markets and have very different feature sets, the Predator at tec divers, the Luna at rec divers who want lots of add on features. What kind of diving are you going to be doing and which features best meet your needs? The Predator can easily do multiple gas switching but doesn't have integrated air; the Predator can manage both open and closed circuit (rebreather) diving but doesn't have a built-in compass, and so on. Look at the feature sets and decide which you want and which you don't.
Personally I chose a Petrel (the model that replaced the Predator) because of simplicity of use and outstanding display, even though I've never used more than two gasses on a dive (yet). I do see deep, multi-gas dives in my future, so I have that capability. I've got an SPG and a compass, and prefer the simplicity of the Petrel without display clutter (e.g. heart-rate monitor). And no matter how good the Luna's monochrome dot matrix display is, it will never be able to match the Predator's OLED display.
Personally, I wouldn't worry too much about warrantee; Shearwater has an unblemished record of after-sale support; I'd be surprised if they'd leave you in the lurch because you bought one second-hand, unless, of course, the seller ran over it with a truck or something and isn't mentioning that to you; always a risk buying on line but there are payment options to help mitigate that risk.
There are other rec computers in roughly your price range that have color LCD displays that might be worth considering if ease of visibility is a primary concern.
