Ortlieb really are the daddies. I can't recommend them enough. I don't know anyone who has got a pair who doesn't rave aout them. They are 100% waterproof and very well made.
I would say that I would always buy Ortlieb's but to be honest it is likely that once you've bought a pair, unless you lose them, they will last you a lifetime. I've had mine 10 years and apart from the odd minor scuff they look the same as they did when I bought them.
If you are going to use pannier regularly getting Ortlieb's is a no brainer really.