When shopping for a shield there are a few things you need to consider. This comes from 20 straight gens as a shielder.
Weight: Not such a big deal for movement speed until you get to the REALLY heavy shields. What it is important for is to stop you from getting block stunned. Yes, you can be blocked stun while carrying a shield. With a weight of 6 or less, there's a LOT of people (on NA at least) who will be able to block stun you if they hold their attack. With my 7.5 weight shield, I only get block stunned by str builds using a bar mace who are holding attacks. I even stop the mallet from crushing through some of the time.
Body Armor: Not really a huge deal. The body armor of most shields is pretty similar and your shield skill does more to mitigate damage in my opinion. It's also rare that you have a crossbow user who can penetrate shields anyway. I maybe run into 1 person a month who can penetrate my shield from close range.
Hit Points: Really important. This is what lets you take the hits. Also good if you take a stray axe hit. If a high-armor, low-hp shield takes a stray axe hit, it's going down VERY soon. If a low-armor, high HP one does, you'll get a bit more life out of it. Still, always put away your shield when fighting a shield-breaker.
Speed Rating: Despite what people say, there is a HUGE difference between speed ratings ESPECIALLY when ping differences are considered. If I see someone with a slow shield, I will move around and spam the shit out of them, 'cause I know they won't be able to react fast enough. 100 speed blocks as fast as a manual block. You can't go higher than 100. Anything less is slower than a manual block. Shield skill does increase your shield speed also. So, if you have a 100 speed shield, you're wasting points. Remember, just because your shield is in the "up" position, doesn't mean it's blocking.
Shield Height: Not really a big deal, but kind of is. I'll explain that more on the bottom. For most things though, if you see a ranged guy aiming at your head, move your shield up. If he's aiming at your feet, move your shield down.
Shield Width: Big deal. It's WAY easier to hit around a narrow shield than a wider one. A lot of times when people think that someone hit through their shield, it's because they're using a narrow one (or a slow one). With a narrow shield, you REALLY need to watch your facing. You can get away with more with a wide shield.
Now, here's where shield width and height make a big difference: the forcefield. The forcefield increase is percentage-based. Therefore if you have a smaller width and height, you get MUCH smaller returns on your forcefield size.
When you factor all of the above together, it comes down to the fact that the overall best shields in the game are round ones. They have the best coverage, good speed, good weight and good hp. (Not counting the metal ones, 'cause those are shit in my opinion)