Upgrade cartridge or turntable?

As a frustrated and another former  Rega RP3 owner, I would unabashedly recommend a different turntable.  Aside from any isolation issues, the lack of an easy VTA adjustment for different cartridges is just plain arrogant.  Some <$600 turntables have a VTA adjustment process with simple set screws at the base. Some carts will not even mount due to their own height.  

There are tons of <$2000 tables out there Rega.  BTW, the Ortofon 2M Blue is quite good IMO, but the market for REALLY good, fairly inexpensive moving coil cartridges has expanded rapidly in the past few years.  

Get a new stylus and save more money. Let us know when you get to $5000.

I'd throw a Dynavector 20 XL or whatever suits your budget on the table - the Planar 4 can definitely handle a big cartridge upgrade no problem.

According to expert advice I ignored in my youth, turntable first, tonearm second, cartridge third. I ignored this advice, to my cost. For one thing, turntables and tonearms don't wear out.

Another thing to consider is bang for buck. There is a tonearm which is within your budget which punches far above its weight (and cost): the Trans-Fi Terminator (yeah,  I know, but I didn't name it). Air bearing sled, and solid, intuitive, repeatable adjustments just like tonearms costing 10 or 20 times as much. I have two of them, running MC cartridges with an air bearing TT.

Put the balance into a record cleaning machine - I like ultra-sound. Works for me, but YMMV.

Get a new stylus and save more money. Let us know when you get to $5000.

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According to expert advice I ignored in my youth, turntable first, tonearm second, cartridge third. I ignored this advice, to my cost. For one thing, turntables and tonearms don't wear out.

Mike and Terry are right. The challenge is making the leap to buying turntable/arm/cartridge separately, when the combination you might ultimately want adds up to $10k or more. 

The tried and true approach is to find the very best table you can afford, and run it with a budget arm and cartridge, until you can afford to upgrade to a really good arm, and then finally cartridge. 

This way you spend the big money on things that really just do not wear out, so they are keepers and nothing is lost in the resale/upgrade treadmill. 

Another beauty of doing it this way, its almost impossible to find a turntable or arm that expensive that's not really, really good. VPI and Rega can sell based on ads. Nobody bought an Origin Live or Graham arm based on an ad. Their "advertising" is word of mouth. 

Take your time. There's a lot more going on than just sound. Another big factor with turntables is the way they look, feel, and function. Pay attention to all those details, plan out your path, take your time.