Check size, ease of maintaining temperature, jets, level and bacteria. I found that a temp. of 101F keeps the water hot enough to kill the bacteria that causes the "scum" on the edges of the tub; also, ease of maintaining chemical balance.
One of the most important things is safety from bacteria. You want to be able to keep it clean without devoting a large part of your life to the effort. After that you should consider how efficiently it heats itself up, as this will add up over time in electricity costs.