The rotary cutter is identical to the cheapie I just bought though supposedly a different brand. For the price I was pleasantly surprised. It's fine. Never heard of that mat company.
Effie is right, I'd go for a decent pair of scissors/shears. I got a set of 3 last year at Aldi that is much better quality than the price I paid. Only cut fabric. I'd also get an 18" to 24" ruler. If you go the rotary cutter route you'll need a plastic quilter's one to keep your cuts straight. With scissors it is handy for drawing a straight line to cut along so you can use a good condition wooden yardstick or a metal one.
To start you don't need fancy specialist tools. Scissors, pins, sewing machine in good working order, thread, fabric and pattern would be helpful. Everything else is luxury and can be added as you go along and build your skill as well as your interest in sewing.
No advice on denim. I'm in North America.