It is a difficult one. I have been toying with both for a long time.
If it was just for me I would have the M all day long, but SWIMBO is a massive consideration I dont really want it to be locked in the garage only used once and a while. Wouldn't want to be too precious over it and be happy with her driving it and taking it out so it doesnt become a barrier where its my car and its a risky thing her grabbing the keys and going to the supermarket etc
I want her to be involved come on meets and continue a tradition of a Euro roadtrip every couple of years.
I know for a fact she would hate the Ms suspension and gearbox, especially the gearbox and the thought of missing gears and rough changes makes me cringe. Even the best drivers struggle with the getrag to start with, although I loved it. The Ms are fun on a weekend hoon but not for traveling long distance imo. My M also had H&Rs and although fun to drive it was absolutely spine shattering, to the point and ex flat out refused to go out in it (that lasted long

)
Just moved around 100 miles from my home town so we spend a lot of time driving back and forth seeing friends and family on the weekend. When I had my M I had a long distance relationship and the M wasnt the best steed for the task.
The 35IS makes a lot of sense and is so comfortable and with the torque it has makes quite a sleeper on the motorway and I found the M great but you do have to wind it up a bit when your cruising.
At the same time loosing money is a worry and the 2011-12 plates with decent mileage are sitting around 22-23 and for a non M car at 5-6 years old I think they are bucking the trend and are on the more expensive side. For example a similar 335I would be around 15k and would probably have a better spec as not many 35IS were made and not many were speced highly. The 35IS is less of a weekend car I think so the toys are more important.
We took a 35i for a test drive and swimbo fell in love with it. Problem with the one we test drove was it has 12k miles on it and the interior was such a mess the leather had warn really badly there were scrapes and scratches everywhere. It may have been down to the fact it was red and shows wear more than black for example if not looked after. The aluminium trim had marks all over the interior door handles and the center console. The car had come from Guernsey so I assume it was due to small trips getting in and out of the car a lot but was disappointed in the quality, it could have been sorted but there was no room to manover with the car as it had to be imported and re-registered, so i walked away and someone else bought it.
Similar with the M the interior, it wears poorly but my previous M with some looking after fared well in my ownership. The nappa is also so soft and lovely, loved the seats. The 35IS wasnt quite as nice as I think it has Dakota? Not sure if a nappa was offered apart from in the white pack? Probably wrong but not many have it if thats the case.
Lots of things to think about both are excellent but as I keep saying it depends on your use case and a compromise has to be met.