My question is which lens should I use
That's like the cake maker asking which mixing bowl should I use. Come on man, you've shot both these lenses. You know shooting an 18-55mm is a whole different world than shooting an 55-250mm. You'll need to use which ever is appropriate for what you're trying to capture. We've not seen the venue, or know what kind of access you will have to the subject matter, or even how large the subject matter (wedding party) will be. We don't know what the lighting will be like. My best pointer would be to make sure the client absolutely understands that they could end up with nothing usable. If they are OK with that, shoot with confidence. I would use the 55-250 when you need to get in tight and the 18-55 for the rest. Bounce your flash as much as you can. Make sure you use a fast enough shutter speed (at the expense of iso). Sharp and noisy is better than blurry and clean. If possible, educate yourself ahead of time to the proceedings. If not possible anticipate them and position yourself accordingly. Take charge, but don't be overly intrusive.