My opinion:
Buy a pci TV card for your mediator and connect the composite/SVHS output you may have on your Amiga to this card, so you can switch by software, and be capable of using a Picture-in-Picture aproach, so that you may even see both outputs at the same time thru the mediator gfx card!
The only downside is, of course, that the quality of the composite/SVHS is not as good as RGB, but then you will only use native modes for demos and games which dont require great image definition. On the other hand, you would avoid spending $$$ on the indivision.
So this is for me the best solution.
PS: You can buy at an affordable price a composite and SVHS adaptor for amigas at
http://www.amigamaniac.com if you dont have these outputs