We are all different, and so are the t-shirts.
Each product has a slightly different design, so please check the sizing charts and measure yourself… the chart's are pretty accurate, so if you do this, you should get the sizing right.
I like my short-sleeve t-shirts quite fitted so I wear a SML or even XSM, and I prefer my long-sleeves more oversized, so I choose a MED or LRG.
It's entirely up to you.