We actually have similar stats.
I'm 5'2", 110 lbs and 15% BF
I also thought I'd be too lean to have them over the muscle, but the doc took in consideration how much breast tissue I already had and the elasticity of my skin.
I had a C cup before losing so much BF, so he knew the skin had room to stretch a little more.
I also told him I'm an active weight lifter. He said that my pectoral muscles are very developed so my best bet was going over. When you do them under, it's not adviced to do a lot of chest work. I really don't want my workouts to be restricted. Going under also means that whenever you flex your pecs the implants are going to do this weird motion that isn't very attractive.
There are down sides to choosing over though. They look less natural, higher risk of capsular contracture and higher risk of ripples when leaning forward. This also depends on the type of implant you choose, size and quality of the skin.
I went with Natrelle Inspira SRX (smooth, round, Xtra full projection) silicone implants, 400cc.
The PS used an inframammary incision.