The laws are changing January 1, so I'd recommend buying the high back booster, as they'll need to be in it until 4'9" AND 8-12 years of age. I've also heard that one law may be changing to require a 5 point harness until age 8, but I haven't seen anything to substantiate this, and I think THAT would be a little overdone.
For the anchors, it's combined weight of the seat and the child, so you'll need to check your VEHICLE'S recommendations and limitations - unless your manual says to defer to the car seat.
That said, I've always used the LATCH system when I can. I know they say that seat belts are just as safe, but some cars we've ridden in are older models that don't have locking seat belts. I'm not sure how they can say that's just as safe. My son is currently 7, and my daughter is 4. Son is unfortunately and begrudgingly going back into his high back booster that will be LATCHed, and my daughter is LATCHed in her seat with the 5 point harness (and will be for quite some time; my son was harnessed until almost 6).
Some of it is so confusing because the side of the backless booster clearly says ages 4+ and usually 40 pounds can ride in a backless booster. Personally, I feel that's unsafe, especially since the seat can slide. Most backless boosters do NOT have the anchors, and I just don't feel they're safe enough. My two cents.