That actually might work. If you use Talkie Toaster tutorial for creating the skeleton of a creature and make a script using the setplayerskeletonpath obse function + force equipping the creature meshes for the certain spell time. I might be a lot of work but would make it for a near perfect creature polymorph spell.
The down side to that is you can not set the 1st person skeleton so no matter what that view is of a human wearing a suit of armor that is of a creature skinned to a creature skeleton not the 1st person human skeleton which equals all kinds of mesh errors, going the OBSE route if absolutely easier as it only really requires you to define the Creatures Mesh as an armor and create a spell script that does all of the changing -> there is a problem though if the player is equiped a weapon class that the creature you just turned the PC into and that creature does not have any anims for it -> bad S**T will happen. probably best to at first restrict the player to H2H then you can script indivitual restrictions on a per creature basis.
The obvious best part of this route is its an on off situation you do not need a quest script running constantly checking for Key Presses.
The good -> Sneaking works.
Down sides are equally obvious and that is you need to script an a$$ ton of restrictions to keep things playing right and avoid constant game crashing, unless of course you want to create about 5000 creature animations to fill out all of the animations that critters are missing.
Going the Mount route has its own down sides -> mainly its a TON of work only made slightly easier by using Andrew Maneri's Animation Wrapper
Besides that mounts are not allowed to use the Left or Right animations while the player is mounted on them (you can call them by script though !! use OBSE to detect if your pressing a button you set to be the new straife buttons, I did this as a test for my AM mount mod and it worked out quite well I used the Q and E buttons as my straifers and left the default controls alone as I kinda like them too just wanted the ability to straife).
Also you would need to create a mounted combat set of scripts (these are easy as pie with OBSE especially since your doing it for critters taht have a known set of attack types/weapons available).
The Good side it will not screw up the 1st person view like going the OBSE set player skele route does.
The Bad -> No Sneaking while mounted.