I'm currently in the throws of the same decision. Been comparing the Tortuga with the Axis, as well as the Genesis Tropic (Sherwood and Genesis are the same company). I've also looked at the Oceanic Excursion. Aside from the basic form/fit/function, which I think is a strictly personal choice - the main difference I'm seeing is the lift capacity. The Tropic and the Axis seem to be designed for lighter duty warm water diving - little to no wetsuit. They both have identical light to moderate lift capacities that vary with size - 23/28/37/42 lbs respectivly for S/M/L/XL sizes. My son is 6 foot and 132 lbs - he wears a 7mm farmer john here in the cold waters of Lake Travis. The 23/28 lb lift capacity of the small/medium is just not enough.
If you move to the Tortuga, you are looking at a much heavier duty, but not much heavier in weight unit. It has a 32 lb lift for S/M and a 46 lb lift for L/XL. These are much better numbers for thick wetsuits. The Oceanic Excursion provides a constant 45lbs over all sizes.
What I have been able to gather - the Axis/Tropic are for warm water, with a minimal (<3mm) wetsuit. The Tortuga/Excursion are more robust year round units for cold water wetsuit (5mm +) use. Oddly enough the Tirtuga is still considered a lightweight travel friendly unit by Scuba Diving Mag - they selected it as a testers choice. As for us - we're most likely going to go with the Tortuga's.
