So which shield would be better suited to the cobra? and how could I tell which is the best one?
Okay, so you'll need to get your map up and do a search for a high population, industrial station. Here you will find a good stock of defence and offensive equipment. As others have pointed out here, a good A Integrity Rating/shields going to knock you back 500k or so (Don't confuse these letters with Class sizes). You can stick a class 3 on a 180 ton Cobra but you really want to look at the optimal hull mass and Max hull mass ton range the shield covers. Class 3's are 165 optimal up to 413 ton maximum, then you have the power draw, the weight and the grade to think about, so it really does depend on your setup.
You may have had an Class 2 or 1 Integrity/Rating A installed, that was why it went down so quickly in a fight (you can install lower (weight) rating shields for extra jump). You can install these lower classes if you are not completely understanding what you are looking at stats wise. So, get yourself a good Class 3 for your Cobra, doesn't have to be all A rating, it'll knock you back for about 200-300k, you won't be having these problems.