Hi jon_fl,
I have a PC 7336 with the 6oz weight and, supposedly, this allows me to use a 6" backing plate (flexible) and I can use 6", 6.5", or 7.5"pads. Lake Country Mfg. pads are the first that I've tried and they get mostly positive comments here at DC & other BB's but it's really a matter of personal choice.
The 6" are flat & I find it advantageous to have as much pad contacting the surface as possible. Also, these are good pads to learn on and they remind you to keep the PC parallel to the surface.
The 6.5" are "cupped" and this supposedly reducing product sling but it also reduces surface contact unless you beardown on the PC a bit. I'd advise against this until becoming proficent (I'm not) so let the PC do the work.
The 7.5" are cupped and the edges are beveled, making it easier to get into some tight spaces like around mirrors & spoilers. Also, the backing plate fits into a recessed area in the back of the pad which protects against accidental contact with the paint.
The 7.5" also seem to have a more secure velcro-backing-to-pad, it just feels like it's made to last.
But .......
I've read that the PC 7424 usually comes with a 5oz weight and that limits it to a 5" backing plate and 6" pads.
I'm sure there are many here at DC that are using the 7424 with 6" plates & larger pads, but I've read that combination can lead to above average vibration & fatigue your arms quicker.
Other comments either confirming this thought or disputing it are welcome, you can't always believe what you read elsewhere.