Batteries are the status quo. Compressed gas is fun - recoil and noise - but the magazines hold less pellets, they weigh more and in cold weather, the gas pressure sinks and the range diminishes. Plus, loading the gas magazines can be time consuming.
Electric rifles are largely superior. You can install batteries with a higher voltage - thus increasing rate of fire - and with a higher amperage - increasing lifespan of the battery. You can also, on some rifles, include blowback kits. They're nothing amazing, but quite a bit of fun.
Also, look at the body of the gun. Most are high quality plastic but quite a few are now metal, which feels more real and increases durability. Also, the quality of the metal can differ. STAR SA80 (ak. L85) rifles are made from stamped steel, very nice. However, Classic Army MP5 rifles are made from pot metal - not so good.
You can always upgrade the internals of electric airsoft guns, with metal motor parts and so on.
Finally, a word on make. TM (Tokyo Marui) is the one to go for - they're tried and tested and they also have the widest collection of models. ICS and Classic Army are a tier up and will put you back by £10 - £20 a gun, but they are SO worth it for the metal bodies and superior build quality.
STAR is a new contender and so far their biggest sellers are the British SA80 rifle suite. They're a very nice quality, but if you want one of these guns, consider ARMY brand rifles. Same build quality, considerably cheaper (I've seen the two side by side - no way to tell which is which!)
And of course there are the cheaper rifles, which are coming on in leaps and bounds, including CYMA, Galaxy, BE and JLS (too many to count!)
If you want any advice, feel free to give me a PM with the rifle you're thinking of. I'll answer to the best of my knowledge
If you're looking for a sure fire winner, I have a few ideas including extras, should you want to splash out just that little bit more (some extras are a necessity to fun playing - such as a couple of high capacity magazines and a spare battery; some are frivolous - such as night vision and grenade launchers.)
Then there are handguns you may want to look at...