BLT studios are on the smaller side. The 1BR and 2BR villas are actually 3rd largest at WDW (OKW largest GFV 2nd). Here's the complete list by square footage:
http://dvcnews.com/index.php/resorts/grand-floridian/news/2291-dvc-villa-square-footage-by-resort
VGF is newer so rooms have less wear and tear, but they will also cost you about 20% more points on average. With 8 people I'm assuming you are looking at 2BR villas. BLT will give you a 3rd full bathroom too. Not a huge deal, but it comes in handy when you have 2 adults showering in the morning and one of the kids needs to go. The walk to and from MK is a nice benefit, especially at park closing/after wishes. We had the same sized group at BLT and never felt cramped. I've never stayed at GFV, but they look pretty roomy from the pictures too. The balcony at VGF is larger. At BLT the 2BR has 2 separate balconies, one off of each bedroom, but the living room is in the middle and does not have a balcony. There is a door from the living room onto the side of the balcony off of the MBR. At VGF I think the balcony stretches across both bedrooms and the living room. If it's a lockoff there is a wall between the balcony on the 2nd bedroom and the main balcony since that room could be rented either as a studio or part of a 2BR. It's easier to understand with pictures:
VGF:
View attachment 83173
BLT:
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