Monocrystalline panels , often abbreviated to mono, use a single silicon crystal to a unit.
Polycrystalline panels , often abbreviated to poly, use a multiple silicon crystals to a unit.
Monos are supposed to be more efficient than poly, but you generally pay more for them of course.
Perc or Passivated Emitter and Rear Cell, is another technology that is apparently better, which can be combined with Mono is seems, although better than what I'm not sure.
I'd check the specs and look at efficiency ratings, and also calculate the price per W (of that efficiency).
Then look at the size of your roof, avoiding obstacles and it will probably narrow the choice down a bit, get the most efficient panel that makes the best use of the roof space.
I'd get larger, better panels now, and plan to add to them later if you find you need more capacity. Buying a cheap panel that wastes space would be the worst as you'd probably bin it later if you wanted more panels in the future.
The price variation for suppliers was also quite extreme when I was researching, I ended up with some much more efficient Victron panels (with a much longer warranty) that came out cheaper than some unbranded units just by switching between a reputable/popular supplier.