You absolutly MUST get a 128 gb, why? you should account for system growth in size and many many programs/drivers that only install themselfs on the C drive.
I have a 128 gig and i have 25 gig left, no idea why but all i did was install ALL drivers (including the ones that do allow to be placed on D).
Your PC just wont be that premium with a 64gb, you will have to constantly look out for HDD space while with a 128gb its allot chiller, afterall...you want an SSD for that premium feeling.
So in shirt, 50 pounds wont get you anywhere, i recommend samsung 840 Pro which is a reliable SSD and has nice speeds, cuzz in the end it must be reliable.
A normal 840 is fine too, just has lower writing speeds, but you wont be writing that much, although it might be a noticeble diffrence. I always preffer to just go that extra bucks to get the nicer version.