I did my instrument rating at Galvin (BFI) and was a Diamond-G club member for five years there. I am now at Pro Flight (RNT). The basic breakdown is that Galvin has much newer equipment, mainly G1000-equipped DA40's and DA42's, though you can also still fly one of their late-model C172's with steam gauges. If you've got money to burn, they also have a Columbia 350, a Bonanza, and one or two newer Seminoles. They have a nice DA42 sim on the property and (I think) an older Frasca 142. Galvin employs quite a few CFI's so there is usually no problem with scheduling rides and ground schools. You pay for all the bells and whistles at Galvin, though.
Pro Flight has older 172M's, no sims, and a less fancy office. They employ fewer CFI's who might not always be as flexible schedule-wise. On the other hand, their rental rates are more reasonable and the mom 'n pop feel is more personal and inviting than I found at Galvin. Renton airporty is not as exciting as BFI. There aren't as many 777s, 767s, and private 757s buzzing around. There's no UPS, ABX, DHL, Ameriflight, AirNet, life flights, etc. However, the 737 line is fun to observe and you get the Class D experience at RNT without having to constantly worry about ducking under Sea-Tac's Class B all the time.
Bottom line, I liked Galvin and I do miss it. My wallet doesn't.