I enjoy Renegade X, more so than the original Renegade. As a frequent player, i sometimes find myself thinking, it would be really awesome to see these W3D mod communities create their mods on UDK.
The benefits of developing on UDK far out way the cons.
You can attract a larger player base, attracting old and new players alike. Totem arts plan on releasing RenX on steam once the game is at stable and balanced state. When this happens, the potential for an increased player base is likely.
There are more developers for UDK than W3D, so chances are you could have a larger team to develop a mod. I know RenX is planning on releasing map tools in the Beta 4, and i'm sure i recall reading on their forums, they're open to other mods using RenX to mod from.
You can take advantage of a superior engine to great effect, more so than you would be able to achieve using W3D. Imagine reborn with better physics and mechanics you could gain from utilising UDK.
It's also visually more appealing. I prefer gameplay over graphics, but if you can have great game play with awesome graphics, why not? As much as some of the developers here enjoy modding on W3D, i'm sure when they release their mod, or patches, they'd like to see it played by many players, rather than a few players.