I have used pentax and nikon, dont like canons menus, the cameras systems all have there pros and cons, pentax is a great camera as are all 3 but the pentax others in body motor so screw driven lenses can be used and lenses with motors built in, also it of built in sr(shake reduction) which works with any lens that is put on it, the down side is that pentaxes longest current lens is 300mm, but 3rd partys like sigma make longer 500mm.
the the nikon 5100 is just as good, but does not offer in body focus motor so lenses with a motor in must be used to benifit from auti focus, else your in manual focus, also you need lenses with built in image stablisation(os/vr/vc) and not all lenses have this, so some unlike the pentax will have you image stablisation, but nikon do offer alot bigger range on lenses as do canon, the canon is pritty much the same but all canon lenses have built in motors for autofocus(i think could be wrong) and like nikon not all lenses have image stablisation,
all will do the job, but i think its worth going a playing with all 3 if you can.