Your shield passes my "keep this rare?" check because it's better than Moser's, so I would definitely bother with the question about upgrading it
At this point, I would upgrade it to a dragon shield, but probably not to a monarch yet. Apart from the additional level req increase, a monarch has pretty high strength reqs as well. You can wait with a second upgrade once you've decided which character will keep it. Besides, rares can be upgraded upgrade it to elite on non-ladder as well.
If you intend to sell it, I wouldn't upgrade it at all, as that won't increase the sale value on average, except you want to take a gamble about the new base defense.
I had already upped to dragon as I had the mats to hand - I agree, I will wait on monarch. I forgot rares could be upped on non ladder. I pretty much only play ladder but do keep some things after ladder ends, so thanks for the reminder.