You don't need either to get offers (except medicine). The extent that they help depends on the course and university. Some unis don't care at all about voluntary work while others might. Work experience, especially in a related field will be more valuble than volutary work in general.
They're not really two separate things as you've put. Voluntary work can be work experience - in which case, great! Or you could be volunteering at a railway which has nothing to do with your application and is something people would do as fun.
But either way, you shouldn't rule out one or the other. Both are exceptionally hard to get, so just apply for everything and see what happens.
Oh right, yeah i've been trying for a placement since early march and still haven't any luck. No dental practices seem to be enrolling that many candidates for work experience and finding a place for volunteering is just as hard.
Well, can you apply to a wider area? Maybe if you got a part time job and earnt money you'd be able to do a week somewhere else and stay in a cheap BandB - hell, even go camping! Or you might have friends or relatives you can stay with. You'll also be able to put your part time job in your application for work experience, which will make you look more trustworthy as you've managed to hold down a job.