you'll get better pictures from the dslr, but you'll have to keep an eye out for it, if it switches off due to it's recording limit (or overheating).
as handsome pete said: neither is a really good idea. you won't be able to cut anything, because you'll only get one shot length this way. also, audio won't be usable.
you could get someone who knows what he's doing to operate the cam. if you want to do more than a clip show cut to music, position some recording device, mic, etc.. near the couple.
or you could put the dslr on a tripod for a fixed shot and record some different shot lengths in between your photographs (say 10sec. length each - countdown in your head) to have some material for editing.
or, get the gopro, position it for a wide angle (but use the normal FOV setting to avoid that fisheye look) and position the dslr for a closer shot of the couple. position both cameras so that the horizontal angles of the shots are at least 30 degrees apart. e.g. in a church you could position the gopro on the balcony looking down and the dslr on an angle behind the altar, to get something like a full or medium shot of the couple.
get people to play around with the gopro before and afterwards to get some additional footage, maybe do some shots with your main dslr (establishing the location with a wide shot, close ups of the parents, guests arriving, throwing of the bouquet, wedding cake, etc....). keep in mind, that the gopro's battery won't last longer than an hour, so get a spare one (or the battery backpack, that'll double your recording time).
still, you'll either have to sacrifice a few photo-oppurtunities to get a decent video or have a pretty boring video that only cuts between two fixed angles. i'd rather concentrate doing either video or photography than doing both half-assed.
