Since turntable systems all work through vibrations, you want only the right vibrations getting to the cartridge. If the deck and arm are adding vibrations, the cartridge will pick them up. A better cartridge will more easily pick up those vibrations, as they can't tell which vibrations are the right ones (from the record groove) and which are the wrong ones (from the deck and arm). With an inferior deck that adds vibrations, the result is degraded sound.
Following this logic, you'd want as good a deck and tonearm as possible, as garbage into the cartridge = garbage out of it. I'm not calling either of your decks garbage by any means, but I'm not sure it would be worth your while to go all out with a cartridge. It'll most likely be similar to a great set of speakers ruthlessly revealing all the flaws of an inferior source or amp.
I wouldn't spend much more than the cost of the deck and tonearm on the cartridge. I have a Pro-Ject 1Xpression with Speedbox II and acrylic platter, and run a Dynavector 10x5. While demoing the cartridge, I got to hear the shortcomings of the deck. It's a great match IMO, but the 10x5 is capable of better performance. One of these days when I have more disposable income, more vinyl, more time to listen to it, and my daughter is older than 14 months, I'll buy a better deck for the 10x5.
