Should I upgrade?

I would say every step will get you closer to the music.It just depends on how much money you want to spend.The cartridge is the 'tricky' part.---It may or may not go well with table/arm of "your" future.
I remember having a Sony DD table about 27 or so years ago. I had my table/arm fitted for a Fidelty Research cartridge and bought their step up phono preamp and was in heaven. You are at a spot where there are many choices and just about all of them will get you closer.

What sonic qualities are you looking to improve? What are your sonic priorities? What is the rest of your system?

Do you suffer from any kind of acoustic feedback or woofer flutter

Have you ever oiled the bearing of your Denon? According to the manual for my SL 1210 (another DD TT), the bearing should be oiled about every 2000 hours (which would be about 2 years for me).

As Piedpiper asked, What do you hear (or not hear) in your current rig that you'd like to improve? That would determine what to do next. Some kinds of things can be improved with maintenance, vibration control and isolation, a better mat, or a better cartridge. Only then would you reach the limits of the drive system or tonearm.

Given that the better DD drive systems are consistent and quiet (and the Denons are definitely counted among the better ones), and a cartridge acts like a microphone when it's in contact with a record, the noise/vibration/resonance control would be the most important thing to take care of first. If you don't and then you upgrade, those problems migrate with you, and they have a profound effect on dynamic range and clarity. In fact, you would want to make sure that's taken care of before upgrading the cartridge, because it'll emphasize the resonances if they're not controlled.

keep your denon

Upgrade to a mechanically better TT.U will eliminate floor noise which will increase with a new and better cartridge.JT

04-16-07: Jaybo
keep your denon

My sentiments exactly. The TTs you mention don't have near the build quality of the Denon you have. I don't know what cartridge you've been using, but you could get a Denon DL-110 HO MC cart and a Creek or Bellari VP 129 phono stage for about the same or less than the entry-level turntables you mentioned.