The SSR studio is about 355 sq ft, the BLT studio about 340. It is a noticeable difference and BLT is more cramped, but, for comparisons, if you've done in the past any moderate Disney hotel resorts (average in the 315 range) or value resorts (average in the 250 range), which many have done with 4 persons, the BLT room is significantly larger. My personal sense is that four in a studio is always too cramped regardless of size but if you are comfortable with four, SSR is somewhat better, but BLT is OK too.
SSR's advantage is that its rooms have recently been refurbished, and it is one the best refurbs
DVC has ever done to any rooms. The rooms are now much nicer than they were before and the second bed is now a comfortable queen Murphy bed that pulls down out of the wall over a couch that folds down when you do so. BLT rooms still have the older design with a couch that pulls out into a bed, for which comfort generally varies depending on person, although putting two children on it is usually the choice that produces lthe least amount of complaints about comfort.
BLT has the location advantage with the ability to walk to MK. Also, along with its sister resort, Contemporary, it has easy access to excellent restaurants and large stores. Moreover, if you get a lake view room and request and get higher floor, you can get very good views.
SSR's supposed location disadvantage has really been greatly overblown. The buses to parks are fairly quick, even though they make five stops in the resort, and you have fairly easy access to Disney Springs with all its restaurants and activities (and if anyone is a golfer, the Lake Buena Vista course is right there). The perfectly kept landscape and lakes throughout the resort also make just walking around a very pleasant activity.