Are you staying on property? Are you planning to rent from Disney or an outside vendor? If both you and your GF have mobility issues, I'd definitely consider getting and ECV for both of you (although, you may have trouble getting them your room). If you both can walk a ways and switch off, you could save money and space. But as an OT, I think whatever device you can use to get out and explore and make the quality of life for you and your GF enjoyable, do it. I'm sure you'll find in searching rather heated debates surrounding the uses of ECVs, but frankly, it's no one's business why you or your GF need them.
We've done both for family members we've traveled with. Of note, we've not rented either since WDW reopened.
We really didn't care for the WDW ECVs but there were some pros:
--longer lasting batter/didn't need to worry about battery dying.
--if there was an issue with the ECV, we could have it dealt with fairly quickly at the park
--speed was set to where you couldn't outrun everybody in your party (although, this doesn't like it applies)
--you could take your receipt from one park to the other and not have to pay for an addition ECV when park hopping--made it nice not to have to maneuver an ECV onto a bus.
--big tires
--larger wt capacity
There were more cons with the WDW ECVs we found:
--you ran the risk of them being sold out
--you couldn't take them out of the park, so you had to walk all over resort you were staying at (they used to have wheel chairs you could "borrow" but that phased out) or any resort you visited for dining, or even walking to the gate of another park for PH
--No option for shade canopy
--long walk from parking lot if you drove (even from disabled parking)
Renting from an outside vendor in the past, we were able to have the ECV delivered to our resort. It was dropped at bell services and pick-up at bell services for return to the company. But I believe there have been some recent changes to that. I think you have to be present for the ECV drop-off now as bell-services won't hold it for you anymore (or something to that effect). We've not needed to utilize the service since the new "rule" has taken effect.
One of the best perks/pros of having an outside vendor ECV that you have it to go anywhere on property, from getting around DS, other parks and other resorts. Some of the cons are getting it on/off ramps Disney transportation. Other benefits are that you tend to pay a lot less by reserving ahead of time. You can often rent shade canopies if needed.
Some downsides, you have to be cognizant of your battery power, making sure you turn your power off to conserve battery power when sitting for parades (currently not an issue), waiting for transportation/long lines that aren't moving, eating--that kinda of thing. They don't always fit in your room (depending on resort type). One size doesn't fit all--if over a certain wt, you have to rent a larger size and pay more (usually). Smaller tires = rail road hazard on main st.
I'm sure there are a ton more, but that's what I came up with off the top of my head. I didn't mention the ECV that we owned that we took for my grandfather one year. That's a whole other "beast"!