I went with an SDG seat (tall) on my yz 125. It was money well spent. Just bolt it on and go. It was top quality too. Besides you'll have your stock seat to play around with.
Both my husband and I went with the SDG seats on our 125s. They have standard sizes tall and low seats. Didn't have to mess with replacing the old stuff. Trying to staple a seat cover back on can be a pain. They are $85 at Bobs. The seat has held up real well, my stock one broke down pretty bad after riding the bike for a year. The SDG has been on for 3years now and doesn't even show any wear.
My stock seat is toast and I have heard of a few SDG seats that tore up pretty quick. I realize that may not be the norm, but it has caused me to re-think buying a complete aftermarket seat.