I own and really like both. The 0550/1 has been my favorite ZT since day one and owning a few models. Had an 0551, thought I wanted an 0560 more and traded for it. I wanted the 0551 again in just a week. Took a while but I traded off my heavily used ZT 0560 for an 0550 and I'm happy.
I don't find a need for super large over built knives very often, as great as they are and can handle everything you could possibly need it for(even abuse in some cases), there isn't really a real practical reason to carry a brick around. I used to always carry a heavy duty knife along with a lighter duty knife. Now I just carry what I need.
Anyways, between the ZT 0550 and Spyderco Paramilitary 2, it just depends on what you're looking for.
ZT 0550 - Tank, nice action, good steel, easy to sharpen, get's really sharp and holds it's edge pretty well, still slices well although it has a thick blade. I have no doubt it could handle anything you could possibly throw at it. Pretty heavy and bulky.
Spyderco Para 2 - Though not as tank like as the ZT it's still pretty heavy duty. The blade is thick but ground flat, so while being a tough folder that could probably handle some heavy use itself, it makes a wonderful and fine slicer due to the grind. It has a more penetrative tip so it's easy to stick and slice with the Para 2. Doesn't weigh a whole lot. Smooth action. The action is just as smooth as the ZT, just add some Benchmade Blue Lube to the pivot and the blade flies out or will over time anyways. Easy to close with a clever lock. Safer. You can choke up on the blade more. Overall, it's one of the best knives out there.
If having to choose for EDC - The Para 2 wins every time. It's more practical, the blade shape is great, the action is smooth, the weight isn't much, the uses vary widely, and it just makes better for EDC than the ZT in my opinion.
I'd get the Para 2 first. You'll likely get both eventually.