Martin Harris's reply:
Firsly - congratulations at being offered a place on two degree courses. Good going!
There is certainly no right or wrong path - it really is just down to your own personal desires, circumstances and skills. Look closely at the course modules and what exactly you'll be doing. There will usually be some singing on an acting course too but of course on a Musical Theatre course there will be much more and the focus will be heavily on the musical theatre side. It's always useful to have as many skills under your belt as you can and if you can sing, dance, act and maybe even play a musical instrument then that will always be useful - versatility often leads to employment.
At the end of the day though, it's your choice - but don't be afraid of getting advice directly from the course leaders themselves and explaining your circumstances and thoughts.
All the best - and good luck with your decision and your course.