There really isn't much of a difference between the two boards. The biggest difference for me was the FireWire on the DS3P + an additional couple of USB ports.
Both will handle 8GB of RAM. I am currently running a DS3P (Rev 1.0), Q6600 (@ 3.6) and 8GB.
Honestly, unless you already have firewire peripherals or plan (very near future) to purchase firewire peripherals, it may not be worth it. I already had firewire peripherals so it was worth the cost to me. You can always add firewire thru a card at a later date.
The P35-DS3R is a very popular board. It's rock solid, stable and very cost effective. It OCs very well.
lol... well, another good thing about the DS3P is the NB/mosfet heatpipe cooling. The DS3R uses a standard heatsink on the NB, so it gets rather hot when OC'ing past 400-425Mhz FSB SDR -- so if you plan to OC higher than those speeds, either get additional or better cooling for the DS3R, or get the DS3P.