Originally Posted by Coldies
Mostly looks. I think as long as the leg supports are underneath the top frame your fine either way. Think of it like your framing a wall. The top of the rig (Top plate) should be supported by the legs (Studs).
If you are welding this dosent make much of a difference. A good weld is generally stronger than the surrounding material.
The 45s will be slightly stronger, slightly. At the weights we are looking at any of them will work it is just what you like to see. I assume you are having someone weld this up for you? If not it might be a good idea to do like coldies says since your welds could be questionalbe.