Upload & Sell: On
If you do a same-day slideshow, I suggest that you include a line in your contract that it is for display that day only, that the file will remain under your control, and that it is NOT a deliverable, meaning it is only displayed that day and then is destroyed. That way, you don't have unedited photos wandering around.
Honestly, a same-day slideshow doesn't too dramatically dampen the excitement of the reveal (so long as you keep control over it), because
1) you keep the five or ten most dramatic images OUT of that slideshow for reveal later,
2) you edit the images so they're quite different when you reveal them later, and
3) people tend to forget the images but remember the fun they had watching them. The day is a blur to most people anyway, so they just don't have enough space in their brains to record the individual images.
I don't do same-day slideshows very often, but when I do, I use Animoto for this purpose. I import into Lightroom, color-code purple the ones I want to use, filter for purple, and export to 720 px on the short side. Then I upload to Animoto (I've chosen music ahead of time), and click create and upgrade to 720p. You need an internet connection, but those files are reasonably small, so a 4G hotspot works fine. Takes a couple minutes to render, but if it's just a 3-minute song, it doesn't take too long (you can reasonably get about 40 images in a 3-minute song). If you try it, I'm happy if you use animoto promo code flojptkv. But to get 720p, you do have to pay for a pro account -- the basic account only allows 480.
The problem with doing a slideshow is that, no matter what approach you use, it will still require at least 15-20 minutes, and probably a bit longer. You can do something while it uploads, and again while it renders, but there aren't a lot of times during a wedding day when I have that kind of spare time.