Hi Rebelyell,
I used to run (did half marathons) and I now cycle instead. I just seemed to be more prone to knee and hip issues which would make some of my back muscles hurt. In the end I took the hint from my body and changed to a bike. No more knee and hip issues.
The bike purchase looks like it would be the main issue but if you choose running, I would go to a proper running shop and have your running gait analysed. The staff can then advise and fit you with the correct type of running shoe (a Godsend if you want to avoid injuries) - be warned, some of those trainers can be as expensive as a new bicycle!!
That said, we are coming up to Sale time so you might be lucky and get yourself a nice bike/trainers at a decent price.
Now that I cycle, I haven't grown a cyclists butt or thunder thighs but I do feel fitter - I can run up flights of stairs without breaking a sweat
... added benefit is it also reduces stress. Yay! You could mix it up with cycle/run one day and brisk walking the next day.
Have fun deciding
Galadriel x