How to choose rose petals?!

You can get them in large bags which is the most economical way of doing it rather than the individual round packs. The only drawback with the large bags is the colour selection is very limited whereas the small packs are available in many different shades to match your colour scheme.

To be honest, a lot depends on how much you are prepared to spend because, as you say, they will be on the floor and most will be dispersed when you walk up the aisle or could be dispersed by guests taking their seats before you walk up the aisle. It's worth checking with your church/venue beforehand too as some places don't allow petals on the floor, particularly churches, as they have to get someone to clear them away afterwards. Similar rules to no confetti throwing at churches!

You'd also need to arrange for someone to place them beforehand - perhaps a couple of willing friends or ask the venue/church if you can do this the night before?