The monstas are great, I had an S4 and loved it, I love the Tard a bit more tho cos it seems to induce the stupid little kid in me and with the very sit upright position suits me well (I have a repaired broken neck full of Titanium)
The Monsta will feel more sporty that is for sure but the Tard is a naughty tool
The down side to it is you will be banned from here and no one will talk to you and you will have to wait outside and

....j/k
Best thing to do is go sit on one and take it for a skid and compare, the new bike is certainly nicely speced so it won't be a step down in performance