Most 120 GB SSDs have half the number of flash lanes as the 250 GB and larger drives, so perform slower. I highly recommend waiting and saving up the extra $20-$30 to get a 250 GB SSD.
Also, be sure you research upgrading the HDD on your exact model MBP. For a few years, Apple was using a proprietary temperature sensor. If you swapped the HDD with one with a different connector or failed to transfer the temperature sensor over, the fan would run at full speed all the time.