Your probably going to end up getting this post moved to the What Bike forum, but till then... on to your question! If you've been out of riding for a while, I think you'll be pretty impressed with how far technology has come. A 125 is quite a fast bike. The only reason I see to ride a 250, is if your pretty good and absolutely in control of a 125 at full power. Or if your a desert rider. Or if your compensating, for, well... anyways!
A 125 is plenty of power for most people, and is definitely easier to ride. They are a bit more nimble, and despite what some people think, can make an excellent woods bike. And with good maintenence can be quite reliable. My YZ has been nothing but perfect for me. In since it being new, its fouled one plug. Thats the only mechanical failure I've had. Having ridden both 125's and 250's, I think its easier to feel at home sooner on a 125. 250's are awesome too, but you definitely have to be disiplined and keep your twister under control, they'll get away from you real easy! So my $.02 would be go 125, its more than a enough bike to have an AWESOME time!
