First lets look at this from strictly a money perspective. Marner currently making $10.9 for 2 more seasons (I know he had bonuses paid at the beginning of the contract, but I will use AAV to make it easier). Once that contract expires Marner will be after an 8 year extension and given his production it will probably have an AAV of around $12 million. So over the next 8 years Marner will collect $21.8 million over the next 2 seasons and approximately $72 million over the first 6 years of his new contract. This adds up to a total of $93.8 million over the next 8 seasons. Over the same period of time Stutzle will make $66.8 million dollars. Thats an additional $27 million dollars over the next 8 years more to Marner than Stutzle.
Secondly lets look at position. 1st pairing RHD and 1st line C are the most difficult positions to find in the NHL. In this scenario the elite winger is not worth as much as the #1 C. Therefore this trade would require the Sens to replace their #1 C when they already have good wingers but no real C depth and for sure not enough C depth to replace a 21 year old #1 C
Prime years - It is understood by most that prime years for an NHLer are 24-29. There are always exceptions to this rule but the vast majority of the time this is the way it works. This would mean that Stutzle still has a few seasons of elite production before he even hits his prime years. The contract he is currently on will have him locked up from 21-29 right through those prime seasons. Marner is already 26, so over the same 8 year span Marner will be getting paid for his 26-34 year old seasons. There is a far greater chance that you will see Marner's production decrease for 3-4 years of that time frame.
Years of control. As of right now the Sens have 8 years of control and cost certainty with Stutzle. Marner's contract expires in 2 seasons. Yes, the Sens could re-sign him but he will be a UFA and Marner will have all the leverage. Trading 8 years of cost certainty for 2 years of control is a loss for the Sens
So given cost, positional premium, top producing years and years of team control - this is a very bad trade idea for the Sens. Even if the Leafs add to Marner, what can they add to compensate for the additional costs, the loss of a #1 C, the potential decrease in productivity and the loss of 6 years of team control and cost certainty? The + added to Marner would need to be significant. I would think it would need to include high draft pick(s) and a top prospect that has potential to replace Stutzle at #1C down the road. So overall I don't see this trade benefitting the Sens in any way.