I'm not a big fan of the small "portable" spas. They have some big
drawbacks. They are usually not as deep or comfortable to sit in, even
if it's only being used by one person. They also use 120V which
brings some serious disadvantages. You can't have the jets and blower
running at the same time as the heater, as it would draw too much
current. So, the power pack won't allow it. This means however hot
the spa is when you get in, it will start to slowly cool from there.
It also takes 4 times as long to raise the temp as it does with a 240V
spa, so you really can't keep it at a lower temp to save energy, then
raise it a couple hours before using it. That's probably not a big
issue if it's indoors, but could be, if it's outdoors. Also, the spa
draws enough current at 120V that it has to be pretty much the only
load on that circuit, which could be an issue too, depending on what
you have available.

IMO, if you can run 240V without major headaches and have room for a
bigger one, I'd go with one of the smaller 240V regular ones. They
only cost a little more, are more comfortable/convenient and the
operating cost isn't going to be too much diff either.