Mobile Network Advice

Having bought a new mobile (pay & go), for which the best deal was to take on the '3' network I now have this phone and my previous mobile which is on Virgin.
Obviously I'm aware of what the Virgin coverage is like but the question is, do I stick with Virgin (just move the sim over) or get my number moved over to 3 - Anyone got a mobile on 3 who can advise on experience ? Or even a move to another provided, the new phone is unlocked so any sim will do....