Your concern about the length of a full tummy tuck scar is shared by many women. If you think about, no patient ever comes into the office asking for a big scar! What patients do say, though, is that they're bothered about sagging of their skin, stretch marks, and bulges of their abdominal wall, none of which they can fix with diet or exercise. Some of their problems are the result of irreversible skin injury which occurs with fluctuations in weight, but some of their problems are the result of looseness of the abdominal wall muscles.
The differences between a mini tummy tuck and a full tummy tuck go beyond the differences in incision length. A mini tummy tuck removes saggy skin between the belly button and pubic region; that's pretty much it. Most bulging of the abdominal wall remains, as the problems with the abdominal wall muscles haven't been addressed. A full tummy tuck can remove more skin and more stretch marks, tighten loose muscles, thereby eliminating bulges of the upper and lower abdominal wall, and re-shape the belly button. While it's true that the incision is longer, in some cases, it doesn't have to extend from hip bone to hip bone. The length of the incision is primarily determined by extent of loose skin.
One more important thing is that most plastic surgeons are sensitive to placing the incision in an area that can be camouflaged easily. I ask my patients to wear a bathing suit bottom so that I can design the incision to fall within this area. This way, even if the incision needs to be longer than a mini tummy tuck incision, it can still be hidden relatively easily.
If you're happy with the plastic surgeons you've seen in consultation, it may benefit you to discuss with them their rationale behind mini-tummy tuck and full tummy tuck. If you're still uncertain after that, consultation with additional board certified plastic surgeons may be in your best interest.
Hope this helps - best of luck.