The I-Trolly pick up point is far away across the street from the parking garage on Kirkman road. Walking to I-Drive will take 30 minutes and a few have reported it's not a fun walk.
It's really a toss up if you are only going off site 2 times and staying for a week.
The rental car would give you freedom to go anywhere you want, when you want. The hotel would charge you $12 to park each night if you get the rental car from the start.
Look into getting the rental car after you arrive. I know the PHB has a rental car office onsite. You might avoid the parking charge that way, because it's the front desk that asks if you drove your own car. The guard at the gate will let you in when you say you're staying there.
A few people who are staying for an entire week plan on calling the hotel to see if they can get the parking fee reduced. If they can, then I'd rent the car for the entire time. Otherwise, go with a taxi or limo service.