You're not really providing us with enough information to give informed opinions. What state? How many square miles does your agency cover? What is the makeup of your area: urban, suburban, rural? What is the geography and topography like: flat for miles around, hilly, or mountainous? Where are the nearest radio towers for the statewide system in relation to your area? Etc.
Anyone claiming "98% portable coverage statewide" is making a very suspect claim. For starters, that figure usually refers to coverage on the street, and often doesn't take into account using the portable inside a vehicle, or even having the antenna jammed up against your body at waist level. It doesn't take into account using that portable inside buildings, which probably accounts for a solid 75% of portable usage in the first place.
Truth be told, if you have solid on-street coverage with the TRS, especially if your area is not that large and you have a TRS site nearby, you can get away with using a portable in place of a mobile. If the radio tower is 1-2 miles away and you have a small, flat area, you're not gaining much advantage using a mobile vs a portable. But again, if you're sitting inside the vehicle with the portable radio on your belt, there's a good chance you'll kill the reception enough to miss calls. At the very least, you'll have to develop a policy that states the portable has to be kept out in the open inside the vehicle. Which of course now presents an "unsecured flying object" issue. And as Dave said, portable batteries die, then you're up the creek without a paddle if you don't have a second portable or spare battery. The mobile radio is much more reliable in that regard. If you lose power to the mobile radio, well you probably don't have to worry about missing a call, since the vehicle will likely be OOS at that point anyway.
How many vehicles need to be equipped with mobiles? Will portables be issued 1 per crew/vehicle, or will each EMT/Medic have their own portable? Again, we start getting into issues of having a backup should one of the portables fail and it's your only portable on the street.
In my opinion, every vehicle should have a mobile radio, and each crew should have at least 1 portable along with a spare battery charger mounted in the vehicle.