I found the following Thetford replacement guide very helpful
Permanent Toilet Conversion Chart
I decided to change from an AquaMagic V to a Bravura. I stayed with the all plastic toilet for weight. You may decide this too when you realize an all plastic unit can weigh under 20 pounds and a porcelain unit can weigh much more than 40 pounds.
Ultimately, you can look at the Thetford (and other brand) web sites to check the dimensions of the toilet. Also consider the arrangement of the toilet flange in the trailer floor.
Thetford refers to direct replacement, recommended upgrade and special upgrade as all being the same mounting flange arrangement. The upgrade refers to whether the new unit has some porcelain or all porcelain.
Available indicates some changes will be required. Most likely the flange will need to be rotated in the trailer floor to accomodate the bolt pattern of the toilet.
Rotating the flange can range from a simple 10 minute task to a major undertaking depending on the type and condition of your flange.
I know this isn't a simple "buy this model answer" but it really does depend on what you want and how much you are willing to undertake for modifications/upgrades.