We don't advise people what size to get (unless they ask us specifically) - they just come back and buy 18fts in large quantities! A large percentage of our 18ft sales are repeat customers.
With regard to microbore versus minibore, there is a very real weight difference. Minibore reels dry weigh quite a bit more than microbore reels and when full of water weigh over 40% more than the equivalent reel of microbore full of water. For some people this will be significant (including myself - my aim being to work as easily and quickly as possible). If you don't lift your reel in and out of the vehicle than it might not be such a big issue.
Personally, when setting up a new system, I would rather match all of my components properly so that everything is designed to work with each other, not having to rely on an electric unit to artificially make it match your set up.
But, like a lot of this game, it's down to what works for you and personal preference which is why we stock both types of hose and fittings.