Yes, you always want to get a SATA 3 (6Gb/s) SSD, even if you only have SATA 2 ports on your laptop or desktop.
SATA 3 drives are newer tech. The firmware on current generation SSDs have better compatibility/reliability than older SATA 2 drives.
No. A SATA 3 capable device, if plugged into a SATA 2 port, will drop down to that speed.
SATA 2 + SATA 3 = SATA 2.
SATA 3 + SATA 2 = SATA 2.
SATA 3 + SATA 3 = SATA 3.