I used to have a Tohatsu 3.5hp auxilliary, and the Mariner 3.3hp will be about the same, light-weight, narrow-width cowl, so perfect for tight space on the transom.
I now have a Mariner 4hp (2-stroke) on my current RIB and it has an extra little grunt but more importantly the outboard saddle is a little more robust. With all these engines they need to be tethered in the up position (with a ratchet strap to a stainless steel U bolt on the transom knee or to the floor) as the tilt brackets are pretty fragile, especially if you're navigating in a lumpy sea and the vibration and shocks you're likely to experience.
Regards reverse gear, I can't remember if the Tohatsu simply rotated around to give you reverse? You'll probably find you rarely need reverse, especially if you're having to deploy the auxilliary!
In my opinion, if you can live with the slight inconvenience of the Yamaha - it's a better engine, removing the cowl to access spark-plug, pull starter rope, maintenance.