Hi and welcome to the Dis.
To rent or not? Personnally, I perfer to have a vehicle. Let me do the pros and cons list or renting vehicle:
Pros to renting vehicle:
* When staying on Disney property (which you stated your staying at PORS) parking anywhere on Disney property is FREE.
* No waiting in line for a bus, no standing on a full bus and especially no hour plus long waits for a bus when the parks close (we have all been stuck waiting for buses after the fireworks at MK and those waits can easily be up to and over an hour in wait time)
* Renting a vehicle allows you to move off property easier to do shopping and certainly dinner (which helps to keep cost down)
* If you intend to visit any other Disney Resort this will keep time down to a minimum as when using Disney transportation, you'll have to transfer depending on which resort you want to visit.
* Having the freedom to go and come at your own whim.
* If doing US or SeaWorld it makes it easier to get there without having to call Mears or hire a taxi (which isn't cheap)
Cons to renting vehicle:
* Cost of rental vehicle (sometimes this can be offset a bit by eating off grounds for dinner). Also some people mention the traffic, personally the only time I run into it is when on the interstate, I have never experienced vehicular traffic on Disney grounds.
Personally, I usually drive and love having my vehicle. We usually go off grounds at least 3 times per trip to go shopping at strip malls, etc. as Disney items are cheaper offsite (ie. got these cute Minnie and Mickey dolls for my vehicle offsite at $3.00 each, on Disney grounds they were $5.75, etc. on T-shirts and a few others all of which had the registered Disney logo on them). We also ate a few times off grounds at night and to tell you the truth, had better meals for less money than eating on grounds. I would tell you to plan out your vacation and if you intend to leave Disney more than once, get a rental.