If you're considering stopping at a Masters degree, don't get yourself into any debt with an expensive program. Masters degrees can be detrimental for hiring, so they can get you in trouble in general, and having one that cost you a lot of money will only make you more desperate to get a paying job.. You may give up on the field just to get your loans/rent/etc. paid.
If you do commit now to pursuing PhD programs, the Masters concentration won't matter much.
Basically, what I'm saying is *I would* do the garden variety Masters program and with the skills you learn there, such as reading literature, networking, etc., teach yourself the foundation in immunology/oncology so you can end up in your preferred field without paying for it.