Depends..For pool I'd recommend the shorter one, the size of a mini-butt type extension in snooker (usually about 6 inches, possibly sometimes 8). These should be made from wood, and will if well made, feel very good when striking the ball. The hit depends both on the quality of the material and the joining to the butt. The new even shorter ones that go between the butt and shaft are also good. Aluminium extensions with telescopic capabilities or longer wooden ones are a necessary evil in snooker, but not in pool. If the mini butt is capable of reaching the shot, I'd never (and I do mean NEVER) use the longer aluminium (or plastic) extension instead. They take away so much feel, it's ubelievable. And the long wooden ones are too heavy. They also tend to mess up my stroke because of balance issues etc. BTW in order of feel: Wood, wood, wood, wood, wood.............aluminium........plastic. Never used a plastic extension that had a good feel to it.
I mean this only as a general comment and not directed at the op, who I do not know, but I get the feeling that a lot of US players overuse cue extensions because the play poorly with the bridge. If I can't comfortably reach a shot with a minibutt type extension I always go for the bridge, and I do feel that this is the correct way to do things from a theoretical as well as practical point of view. The longer ones, then, are used only in combination with a mechanical bridge and never as a substitute for it. I see some people whack at shots with a huge bridge lengths and extensions in a way which is counter productive. They'd be laughed out of the building in any snooker establishment.
BTW I know someone will bring up Earl Strickland and his "spear"...He is EARL F'ING STRICKLAND guys..If you have that kind of stroke the rules do not apply, and he doesn't use it the way idiots use long extensions. He uses it to add length to the cue behind his cue hand, mostly.