I haven't done Turus Mara, but they don't skip Mull. It is impossible to get to Staffa and Treshnish other than by leaving from Mull.
It is shorter than West Coast Tours because the tour leaves from the port of Ulva Ferry, half way up the west coast of the Isle of Mull. That is no bad thing because that part of the island is little visited, and has it's own charms. It is also a shorter day because you are closer to the islands than from Fionnphort or Iona.
Good and bad, good as less time on the boat, bad as you miss some of the coastal scenery on the way up.
Personally I think it would be a huge shame to skip Iona, but if Iona doesn't interest you, then so be it.
Note that with Turus Mara you have to add on the cost of your CalMac foot passenger tickets to Mull, whereas West Coast Tours include those.
If you drove over add on the cost of the car on the ferry, and the gas used in driving on the island. The benefit is that if you took the early boat over you could take the long route to Ulva through Tobermory and Calgary, then drive back direct to Craignure afterwards (BUT be careful with your times, watch the clock, don't dawdle too much). So you have seen more of the island. Don't take the 0955 boat over with your car as, frankly Turus Mara are more skilled at driving on the Island roads than you. They will beat you to the boat!
So several pros and cons to consider.