I'm afraid I'm not sure what to recommend. I wouldn't say that any one of our locks are necessarily "stronger" than anything else we sell. They all pretty much do their job equally well.
It sounds like this might be the wrong solution for the issue at hand. the lock's job isn't to keep the upper sash up, it's to keep the lower sash from being opened. The upper sash shouldn't be falling down at all.
The hardware that keeps it raised are the balances on either side of the frame. Could it be that those have failed? Would that be something you can investigate to confirm? We do sell a number of different balances and one of them very well may be what you need, but I'm not sure. Without seeing your window or knowing more about it this is admittedly an educated guess.
It's possible, but unlikely. The pivot shoe's only functions are to allow the window to tilt in and to lock itself in place when that occurs. It's not involved in the sash actually staying up. That's 100% on the balance. If your top sash is drooping down then the balance should be the first thing you check, even if it's just to eliminate it as a possibility.
