One thing about the Large Manix is that even though it has a bigger blade, the choil allows you to hold it for precision cuts in a very comfortable manner.
A benefit that I can see for the Para and even full sized Military are that they have the fully serrated option.
If you use any of these knives very hard in the way that they were designed for, lock strength should not be a concern between any of them. Stabbing deep into wood and then twisting the knife hard may increase the chances of the liner lock on the Military and the compression lock on the Para to disengage. Hard prying with any of these knives (and folders in general) will increase the chances of the knife failing in some manner.
I'm pointing out the obvious because the real question isn't a matter of
if any of these models will hold up compared to each other, other knives or in general, because they absolutely will (keeping in mind that there will be some defective products in any line from any company in all of business). The real concern for you, IMHO, is what feels the best in your hands. To really appreciate what Spyderco brings to the table, you need to hold them, and better still, use them. Try to find a store near you if you really want the best option for yourself.
All this being said, I likes the big knives. The choice for me would be between the Full Sized Manix and the Military (CF BG42
).