Hi Binki!
I'll try to attach some pictures to showcase the difference.
On 100% poly shirts the design will be very vivid and vibrant and will last as long as the garment itself.
The design on 50/50 t-shirt will look washed off and faded even before the 1st wash.
As mentioned many times on the forums, there are Hanes 50/50 t-shirts - 100% poyester outer layer and 100% cotton inside. The disadvantage (from my point of view) is that they are too thick, they pill and make you sweat.
Textile industry is rapidly adapting to active life-style and technological changes - there are now stain-repelling, anti-wrinkle, moisture-wicking, odour-resistant, temperature-controlling (etc., etc.,...) fabrics on the market.
Most of them are man-made (synthetic), the rest are coated for enhanced performance. To top it off - there is polyester that looks and feels like cotton, polyester that feels like second skin... polyester that defies the definition of polyster.
If you choose dye-sublimation as your preferred decorating method because you want to deliver full-colour PERMANENT designs (no minimum order, no set-up fees, photo-quality prints - you know?) - you shouldn't be compromising it with the quality of your t-shirt.
Like our mum's and dad's told us (me, anyway) - if you choose to do something - do it properly or don't waste you time.
How you will market your shirts - is a different story all together...
Best of luck!