if you've never checked it out before I suggest going on mudmotortalk.com there is a lot of knowledge on that site. But from what I've read and learned is that if you have a modified v jon, I'm guessing thats what you have since its a grizzly, a long tail motor would be best in the 18-23 hp range. Long tails perform better on the mod v than the surface drives do. also a long tail is cheaper since there are less moving parts. If you were to go with a surface drive you'd be able to go with a smaller motor say 12-18 hp and see fairly good speeds out of it but from what I understand a surface drive will perform best on a true flat bottom boat. But if you plan to go with any kind of mud motor make sure you install a grab bar on your boat and always always always wear your kill switch.
Pros to a surface drive- Speed, load carrying, turning radius
Cons to a surface drive- price, not so good in the mud, you'd need a different boat to see try performance out of a surface drive.
Pros to a long tail- Cheaper with less moving parts(now gear reduction being done) excellent in the mud/stumps the long tail allows you to dig in deep into the mud and find something for the prop to bite on
Cons to a long tail- The won't be as fast as a surface drive, they can't carry a load as well, they have a huge turning radius compared to a surface drive, no neutral, long tails will also beat you up and come close to tossing you out of the boat when you hit stumps and things like that the shaft will come flying out of the water and the tiller will do what ever it wants to and that even means giving you a nice charley horse... happened to me.