When Noah was EBF, we used diaper service prefolds and covers (and fuzzi bunz).
The only covers that would hold in poo were bumpys, which have gussets.
For an older baby, I don't think you *need* gussets, but I do think they help to hold things in, for two reasons: 1) the poo has to make it past 2 separate pieces of fabric, the gusset edge and the elasticized edge of the diaper; and 2) gussets, at least on my kid, seem to operate by helping to keep legs from gaping when babe is very active. On this later point, gussets seem to take away some of the "pull" on the diaper leg, which reduces the stress on the elasticized leg so it stays in place better.
I have a few diapers that have different kinds of gussets. Bodinks, which aren't made anymore, has a gusset that runs much of the full length of the diaper (an elasticized straight line) but the outer edge of the diaper actually doesn't have any elastic. This diaper works really well on those occasions when my nurseaholic toddler barely eats. I also have a SleepyKats fitted, where the gusset is styled more like an extra half moon piece of fabric at the sides of the fabric. The gussets are the only elasticized parts of this diaper, which is probably why I don't get a very good fit with it. Also, because the gusset is made out of the same woven as the rest of the dipe, it doesn't seem to have as much stretch to it as it should to prevent blowouts. I haven't had any blowout problems with it, but then again I have hardly used it because I don't like the fit and it is not very absorbent. (It is a clever one size design, though). My favorite gusseted diaper is the Elbee (Laura calls them sidewalls), and her gusset is a half moon shaped piece of lycra fabric. I love the way this fabric hugs my son's legs, softly but firmly. I have put this diaper on him numerous times when he's had runny poos, and those gussets keep it all in. If I were to diaper another newborn, I'd get elbees with sidewalls. I also have a cottontail organics dipe with gussets, and it's at preschool now so I can't check the details, but I remember that the gusset is similar to the SleepyKats, made of the same not-too-stretchy fabric as the dipe. Again, though, I can't remember having any problems with it in general. The only dipes that I have routine blowout in are Fuzbaby fitteds and Firefly fitteds.
Karla