I'd go with an MTB rear derailleur. Two things to look out for: chain wrap capacity and cog capacity. The chain wrap capacity for a road RD would be OK but it might not be able to get the jockey wheel low enough to clear the big cog. And if you can get the jockey wheel to clear then it will ride too far away from the cassette in high gears. The MTB RDs have a steeper "slant" to them that will keep the jockey wheel the right distance when using a large cassette. Generally if you have 11/28 or wider it's better to go with a MTB RD.