Rega P5 vs RP6 purchasing dilemma

Ok, I’ve never posted on this forum before but I'm rather interested to hear what some of you have to say about this. Basically, I've decided to buy a Rega turntable and am torn between purchasing a new RP6 and a (slightly) used P5 being sold online by an authorized Rega dealer. The P5 can be had for $850 or $1,100 including the power supply unit upgrade. The turntable already has upgraded interconnects, Cardas solid core OFC I'm told. A brand new RP6 would be $1495, also including the PSU. If anyone wants to know my current setup (hoping to upgrade in the somewhat near future) is a Rotel RQ-9708BX phono stage, a Harman Kardon HK 3480 stereo receiver and a pair of Kef Coda 9.2 loudspeakers, or alternately a Little Dot i+ headphone amp with Philips ECG 6028 tubes and some Grado SR225i headphones. Also, I'm planning on putting a Dynavector 10x5 on the new turntable.

I've been considering a wide variety of factors in making this decision. The first, and the one that led me to start considering a second hand P5 as an alternative in the first place, is the tonearm. The thing that first attracted me to Regas was the quality of their tonearms, so this is of great concern to me. I understand the P5 comes with the RB700 while the RP6 comes with the new RB303. It seems people have generally been impressed with the performance of the RB303, as it’s already been used on the recent RP3. I can believe that it’s an improvement of the RB301, but it’s a bit harder to believe that it could match the performance of the RB700, especially considering that the RB700 alone sells for the price of a complete RP3. I suppose it’s possible Rega pulled off an incredible feat of engineering and managed to produce an arm the sounds as good as the RB700 for something like half the manufacturing cost but this seems somewhat unlikely. My guess is that the new RB303 narrows the gap between the 300 and 700 series considerably but that the RB700 still has a bit of an edge over it. If that’s the case, it would seem the primary advantage the RP6 has over the P5 is the platter and subplatter and possibly the new “double brace”, but who knows if this really does what Rega says it does. As for the platter and subplatter, this brings me to my next point which is upgrade potential. In reading discussion forums the past few days I’ve become very intrigued by the idea of adding Groovetracer upgrades to the P5, particularly the subplatter one. Now, even if it’s true that a stock RP6 outperforms a stock P5, I’d have to imagine that a P5 with several of these Groovetracer upgrades would beat the stock RP6, and I could put together the upgraded P5 for about the same cost as a new RP6, so that’s very tempting. I also wonder about the upgrade potential of the RP6. Would the same upgrades also work on that turntable? Would it require a different subplatter of a different size or thickness? If so, would Groovetracer be likely to produce a replacement subplatter for the new ‘table? I have no idea about any of this but I sure am curious. But mostly, I just wonder if the RP6 really is a step forward from the P5 or whether I would be better off just grabbing a P5 while I have the chance to get a good deal on one. If the only significant advantage of the RP6 over the P5 is the platter and subplatter, that can be easily remedied and that’s a whole lot more practical than replacing the tonearm on the RP6. Furthermore, it seems the RP6 may not be as accommodating of upgrades as the P5 and that would be a significant disadvantage, but again, I don’t really know if this is the case. Another consideration is that I’d really like to buy from a local dealer and have an excellent neighborhood audio store I’d love to support but could, of course, only buy there if I went with the RP6. So, um, would anybody like to weigh in on this?