How about opening up 2 separate contests in the same theme then; one for HL2DM and one for UDK (or UT3). This allows almost everyone to enter as presumably most people will be familiar and comfortable with at least one of these two, and even if they aren't, they cover the two main approaches to level editors (additive and heavily brush based vs. subtractive and heavily mesh based) so will be comparable to people's engines of choice.
The 2 would be kept separate, so there would be one poll for best HL2DM entry, and one poll for the best UDK entry. This keeps the entries in each category fair and comparable, and if people aren't able/willing to play entries in one engine or the other then they don't have to but can still feel they are making an unbiased opinion based on seeing all the levels in their chosen category.