Hard to say for sure, the HF specs don't quite make sense. But what they say is that the 240 volt model takes over twice as much power as the 120 volt, which presumably mostly goes into the weld; and that the 240 volt can handle up to 3/16" thick while the 120v can only do 1/8". I opted for the 240v, though I had to make up an extension cord to run it from the dryer outlet in the next room.
I've not tried doing floors and rockers; but the short arms and requiring access to both sides of the weld can definitely be hard to work around. I'd be surprised if you don't wind up with places you'd like to spot weld, but cannot with this welder.
Many years ago, a friend of my Dad's (who ran a body shop) was showing off his latest acquisition. This was a spot welder with the tips/electrodes mounted on cables, and he was showing how he could do two welds at once, working from only one side of a panel. Pretty neat! I don't recall now if it plugged into a standard "stick" (SMAW) power supply or had it's own; nor what brand it was. Can't seem to find it on Google, so maybe it wasn't popular, but it sure looked like it would be.