Lower speed counts means wider gaps and wider tolerances, so cheaper to make and easier to tweak.
More gears really just means smaller steps between each, so it's more likely to find the one that suits you at the time and it's smoother moving up and down.
For beginners, less speeds is fine. If you're somewhere hilly you want something with a wider range (like 11-34 teeth) and if you're somewhere flat you want something with a narrower range (like 11-28 teeth) to give you smaller gaps between gears.
I wouldn't worry about it too much - just buy something you feel comfortable sitting on and can afford, then you can upgrade later if you feel it's lacking.