Scott,
as far as I know, the main differences between the RD 700 SX and the RD 300 SX are the different action, the lack of two expansion slots and -most important- the lack of the beautiful multisampled Steinway (I think it's called "Superior Grand"). It's true that the same piano can be found in the new SRX-11 card, just released by Roland), but since the 300 SX has no expansion slots the only way to have that sound would be to buy a Fantom XR and insert the SRX-11 into one of the SIX slots.
As you know, I had been thinking about the same lines: adding a 76 or 88 notes controller to my rig and play the Tyros via midi. I was thinking of the 700 SX because is very thin and this would enable me to lower the upper tier (where the Tyros would be) without compromising the access to the display and the buttons of the lower keyboard, but even so I think that I couldn't reach easily all the Tyros buttons and maybe should buy a pedalboard to trigger things like fills, style variations, etc.
Even most important, I don't think that I would feel at ease having to do things like editing styles or registrations, because ergonomically speaking it would be more tiresome and time consuming.
The experiences I had in the past led me to think that it's much better to have two keyoards on the same level, maybe at a 90° angle. In this case the G 70 too could be an option.
Anyway, if you are interested in reading more on the 300 SX. you will find some user reviews here: RD 300 SX user reviews

You can then compare them with the user reviews of the RD 700 SX
Ciao
Andrea