I've always used a clam style tool keg because I thought it provided easier access to tools. The downside is the zipper portion is vulnerable to weather and my tools end up rusting.

With this one, I haven't had any rust problems. You may want an extra strap to secure it you your bottle cage as mine had trouble fitting on one of three.

Fits a tube, a patch kit, two tire levers, a small bone wrench, extra bolts and links, and a set of individual allen keys.