3 things you consider when buying memory.
Speed, timings, and voltage.
Compare these, if you are looking at 1600mhz memory, look for memory that has the lowest timings, it will be the fastest. Something everyone seems to not pay a lot of attention to anymore, and timings are very important.
Make sure the memory is 1.5 volts. Some companies will push memory voltages up to achieve higher clock speeds, so you want stock voltage, decent bandwidth, and tight timings.
Other than that, differences are cosmetic, or maybe they have heat spreaders attached (they look cool, but you don't need them, and will interfere with some CPU coolers on some boards-most are removable.) the Vengeance Pro series has faster timings that the regular Vengeance. I am not certain exactly which Crucial memory you are looking at exactly, but it may have the fastest timings. Personally, I have the Vengeance Pro because I got a really good deal on them, but if I was buying today, I would buy the Crucial Ballistix.