'Indistinguishable' is much more common, and you should use that. Most people won't know 'undistinguishable' and it will look like a mistake to them, even if in fact it is occasionally used and has therefore got into dictionaries. It often happens that both 'in-' and 'un-' words exist at the same time.

As adjectives the difference between undistinguishable and indistinguishable is that undistinguishable is imperceptible; indistinct while indistinguishable is not distinguishable; not capable of being perceived, known, or discriminated as separate and distinct.