There are minor differences between the M7 and M700 but nothing really strength wise. They are both very stout accurate receivers great for building on when accurized properly. Out of the box it is a crap shoot what you will get ranging from a steller shooter to a dismal one.
No matter which receiver you decide to go with have it fully accurized. This may be obvious to most here but sometimes it is not.
To be totally honest about your 300 WSM
project. If you will be using this rifle for long range shooting or even want to use the tipped bullets on the market, these two receivers will be very short in the magazine. If you want a single shot receiver then either will work perfectly.
For a repeater I generally perfer to get a Rem 700 receiver for a standard length belted magnum such as the 7mm Rem Mag.
This will allow you to use bullets up to whatever weight you want and still use the magazine. Added weight will be slightly more but to be honest, it is not enough to even talk about.
While the short action is in theory stiffer, the long action receiver has a longer wheel base if you will between the action screws and is generally more stabile in a properly bedded rifle.
For the 7/08 in the ultra light, I would probably go with the SA 700 because of the vastly larger selection of aftermarket componants offered for it especially in ultralight stocks.
Your bullet question has been covered well enough.