I'm really surprised that they would even consider making a clamp bar out of aluminum, I have seen some aluminum take quite the abuse but with something like this kind of pressure these are seeing it's just a poor choice on there part. I would definitely spend the extra $4 and get the steel ones.
I mean all of my clamps are steel, and I can still put enough force on them to get them to bend. Aluminum just doesn't have the tensile strength to take this much pressure unless it's 7075-t6 which I'm sure by the price of the clamp that it is not, my guess is it would be 6061 at best which means it has a 40,000psi tops and it being square tubing instead of solid isn't helping in this case, if we were talking about steel on the low end before forging we are already looking at 58,000-65,000psi before forging so it may be much more than that. Sorry for the long rant but in short, I would go with the steel for a couple dollars more.