HIFS offers free breakfast and food discounts at other times. It's a little closer to the Disney parks. It has more of a hotel feel, if you like that. And as a large hotel with many floors and a kid-friendly marketing design, most people report it to be bustling at breakfast and around the pools.
With MGV, if you planned to squeeze the 5 of you into a studio (which I'm not sure they even allow), take the HIFS instead, since it's got separate sleeping space for the kids. But if you'll have at least a 1BR (preferably a 2BR) at MGV, then this is a much more desirable choice!!! You'd have fullsized rooms with complete separation/doors between all rooms. 1BRs and larger offer a well-stocked, fullsize kitchen and dining table to seat 6+, a large MBR w/master bath, including double jacuzzi, an inroom washer/dryer and a nice screen balcony. A 2BR is considerably larger than a suite at any suite hotel. The 2nd BR and bath are spacious with full furnishings, plenty of closet space.
MGV is really a deluxe resort. The main, freeform pool area is really beautiful, with fountain waterplay area, hot tub, separate lap pool and kiddie pool, all overlooking a small lake with boat rentals and a sandy playground and beach volleyball. There is also onsite golf (most rooms overlook the course). There's an onsite (golf) restaurant, a deli-mart, fitness center, indoor playroom with ballpit. The whole feel of the place is quite lovely. It's family-friendly without being noisy.
If the cost is similar, this is a hands-down choice in favor of MGV, IMHO. Timeshare tours offered by Marriott tend to be fairly low pressure compared with the rest of the industry. Plan to spend the alloted time at the tour but it's not bad. If cost is a factor, then you need to do what's best for your family. HTH.
