Any gas pistol costing less than about £80 new is likely to be a big disappointment. If you want something reliable then look for the established makes, e.g. TM, WE, KWA, etc. There are a few cheaper ones available but the likelihood of them lasting is not great. CO2 pistols usually have high FPS and most sites will not allow you to use them anyway, so you will need to look at a normal GBB.
Electric pistols are more of an all year option, as they should work even in the coldest days, whilst most GBBs will vent gas easily and not work when the temperature is low. Electric pistols are usually a much lower FPS (typically around 200 fps), so are perfect for CQB, where engagement distances are short and the mags are usually cheap by comparison to GBB mags.
Whilst a pistol is a useful backup, most people who carry them rarely end up using them, unless their main weapon breaks, they run out of ammo, or they carry a sniper rifle, in which case they need to use it if engaging targets inside the minimum engagement distance (usually 20-30 yards depending on site).
I've got four, a TM and an ARMY G17 GBB, a CYMA Dessie AEP (converted to Lipo) and a spring 1911 (for very cold weather games). The Army broke early but now works fine having been upgraded with a TM slide. The Dessie is huge but has good trigger response from the Lipo conversion, but the TM outshines them all and is my pistol of choice in all but the winter months, unless playing indoors (e.g. The Mall).