Cartridge Quandary, two tables four cartridges.

Here's a good cartridge quandary in which I find myself. I recently took possession of a quite nice Acoustic Research XB. I already have a Denon DP-7F (p-mount) as well. I also have four cartridges I can use. Three of which need new styli. I have an Audio Technica AT450 with a new upgraded stylus, currently on the Denon. I also have a Shure RXT5 p-mount with adapter I could use on either table. The AR has a Pickering XV15-400 mounted but also came with the original Shure M91.

Of the four what are the two best cartridges for the tables I have?

And does the answer change if I stipulate I only want to buy one new stylus, if so for which cart and on which table? The AT450 has adapter and could be mounted on the AR table but only the AT450 and the RXT5 can be used on the Denon, p-mount you know.
I would buy a brand new Grado black cartridge for about 60 dollars new at needle doctor. It is a much better cartridge than any of the old ones u have listed and u wont need to buy a new stylus. Hope this helps PS It is available in both P mount and standard mount versions for the same low price. Shay
I've been looking at the Grado p-mounts. Before I bought the AR table I was thinking about getting one for the Denon. But I've not found a lot of hard data on cartridges in general, reviews and the like. Anyone know of a good place to go for that sort of thing for those of us who want that best reproduction we can get on low three digit budgets instead of four?