FR has the better seat pitch and is generally more on-time.
VY is a LCC/conventional hybrid airline.
I've flown both. If the price is the same, I'd probably take VY, as you can get to the airport later than on FR. While in theory, the carry-on policy is the same, FR's is better if you buy speedy boarding. I've had VY give me trouble over my 1+1.
VY also tends to operate from 'normal' terminals, whereas FR operates from LCC terminals - whether this is a plus or a minus depends on the airport and your origin/destination.
VY's customer service is marginally better.