Am interested if you have that info still to pass on.
Also wondering if there is much diff in using cotton to poly or microfibre (advantages vs disadvantages???? can anyone enlighten me?). cheers
If you are going to use dye sublimation as your decorating method cotton t-shirts are not a suitable option. Sublimation only works well on synthetic fabrics - the higher 100% of polyester, the better/more vibrant print you will get. If you are printing on a blend with cotton - dye sublimation inks will only react with polyester and will wash off cotton (if you are after "vintage" washed-out look it may work for you).
The major questions, I suppose, you should ask yourself when comparing the fabrics - what is your target market and what is your decorating method.
Microfiber shirts/performance apparel/wicking t-shirts are generally used for sporting applications. Best decorating method for them is dye-sublimation because it doesn't impair wicking properties and gives you freedom of full colour designs.
There are moisture wicking polyester shirts that look and feel like cotton - they are great for any active wear and casual application and with dye-sub printing could be easily decorated with full colour over-sized fashionable graphics. Popular choice for printing of photo images.
If you are planning to use dye sublimation, keep in mind - you will not be able to use natural fabrics or dark coloured t-shirts.
Decorating methods for cotton are screen printing/placticol transfers, DTG, vinyl/flock applications and embroidery.