I have the Montane Jaguar and it is a great and very warm jacket. It is not windproof by any stretch of the imagination, but it is significantly more wind resistant than ‘normal’ fleece. I used it in Northern Sweden back in February over the top of a light fleece and baselayer, in around -25degC conditions and it was plenty warm unless there was a reasonable wind when it lost a bit (but still good). If you are going to mostly wear it under a shell, then it’s a good bet as it’s comfortable and warm and wind resistant enough to cope with not having a shell if it isn’t too windy. A Primaloft (or similar) jacket would be slightly more versatile in that it would be totally windproof and could be used in anything but proper rain without a shell – but aren’t quite as comfortable as a fleece. The Primaloft would probably weigh less than the Jaguar (just).
If you are taking a down jacket for the really cold days, then I’d go for a fleece as it will breathe better under a shell. The Jaguar is a good shout, but it is VERY warm and maybe slightly too much if you wanted to use it in active use. Thermal Pro stuff would be the best bet if you wanted to use the fleece whilst active as it is very breathable, but be aware that the wind resistance of Thermal Pro is essentially zero. With a shell on top it is brilliantly warm. Without, then the slightest breeze will strip all that warmth away.