They don't have to be the same wattage, but bear in mind the amps power will be split between them, so if you had an amp that put out 400W into 4 ohms, you'd want two 8-ohm cabs rated at least 200W each.
In general, many 15s are deeper and bassier than 10s, but this is not always the case, it depends on the speaker and cabinet design and also if there is a horn/tweeter.
A decent 1x15 will do you for most small pub type gigs. My favourite setup is a vertical 2x12 though, as it'll cope with larger venues, doesn't take up as much floor space as a 4x10 or a pair of 1x15s, and the vertical stacking arrangement means both you and the crowd can hear you better.