I'd say they are both Awesome in many ways, but they all have their flaws as well.
Phoenix-Storms' props takes a lot away from the Imagination perspective of Garry's Mod. As in: You can make a House out of Fridges and Companion Cubes, OR You can use Phoenix-Storms' props and make a house out of Generic metal squares...
Yeah. But Wiremod has no flaws on any perspective to my knowledge. It fixes every gripe a new or experienced Garry's Mod user may have when constructing things. Need to build a car? No Numpad? Hate to have to use the Numpad keys? Wire that shit up! Want to make a turret? Want it to be aimed by your mouse? Wire that shit up! Need make something revolutionary inside of Garry's Mod, but can't with the limited tools available? You get the drill.
Oh Wiremod and Phoenix Storm, how I love you so.