All are good, all are fast. Compared to an hdd the very cheapest SSD available 5 years ago spanked hdd's and made Windows fast and responsive. The very best ssd today vs an average ssd 3 years ago offer an almost identical experience to most people for most situations. Benchmarks are great, but how your PC feels is almost exclusively down to 4kb random reads, which are entirely latency based, which hasn't changed for 3+ years now.
M500's are still good, if that is what they are called, I can't remember. Crucial ssd's have been incredibly good in a value/performance sense, close to as fast as the best, won't notice a difference but often hugely lower in cost compared to the "top" drives. Frankly I would buy whatever was on offer, from where ever it was on offer and get the best £/gb you can find, you'll barely notice the difference.
You can't really go wrong, I wouldn't go really old. IE anything released in the past 18-24 months you will basically never notice the difference between any of them or the best current drives.
