I've been fabbing sliders for my trucks for about 6 years, building and selling them to friends in the rock crawling community for about 5 years, right up until last year when I quit competing in rock crawling to spend more time with the family.
Square tube limits the ability to make good looking strong corners, you have to weld every corner which makes it a weak point, the corners on square tube sometimes catch on ledges and make it harder to slide off where round has no corners to catch, and lets face it round tube just looks better.
With round tube you can make a nice tight radius curve (on a tubing bender) that retains strength and looks much better.
Never attach sliders to body mounts, it'll introduce forces to the body mount that it was never intended to withstand and it'll fail dropping the cab directly on the frame at the very least, possibly causing body damage or making the vehicle undriveable.
Attach the end of the tubes to 6"x6"x1/4" plates then weld the plates to the frame, this spreads the force/weight across the frame better. Most frames are so thin they would never withstand the stress of a slider tube being directly welded to the frame, they'd break off and take chunks of metal out of the frame the first time you dropped the truck on the sliders (this I've seen many times).
.120 wall 2" steel cold rolled round tube is probably the best material for sliders.
Never use aluminium, even though it saves weight rocks will gouge into it and make it harder to slide across. Steel stays nice and slick allowing you to power the truck right off obstructions even with 2 wheels in the air.
Cold rolled is the best but HWW (Hot Wire Welded) tube will work just as well. HWW is always cheaper $$ than cold rolled (steel prices fluctuate with fuel prices) I've heard a lot of complaints about the possibility of HWW splitting at the weld on impact points, after 8 years of competitively crawling I've never seen one split yet (still wouldnt use HWW for a roll bar though due to other issues but for sliders it's perfectly adequate).
Good sliders should be welded to the frame, I've seen lots of sets pinch bolted on the frame with plates on the inside and outside, this always causes bends in the frame rails.
Always gusset corners and joints if possible, when your finished the sliders should be able to support the full weight of the truck, in addition to sliding over obstructions you may need to use the sliders as pull points or lift points for the truck at some time.
If you need steps on top of the tubes a piece of wire mesh tack welded to the top of the tube works well to get grip.