I would think very carefully before buying the Trakker if I were you! It weighs 5.2 kg!!! That's FIVE kilos.
I can understand you wanting to upgrade warmthwise, but you need to have regard to what you are doing. If you are only camping from a car, then you could take a couple of quilts from home and be warm without spending £150. The Trakker seems to be intended for Carp fishermen, sleeping on a riverbank near the carpark!
Sleeping bag ratings vary a great deal, with some manufacturers quoting ridiculously optimistic figures. I have had Coleman bags before, and mine was not up to properly cold weather. I also have a ME TDSIII, and have to say that you will struggle to be cold in it. I have stopped using mine much as I am always too warm. It is now used by the missus, on the VERY rare occasion she can be persuaded to sleep in the tent. This woman takes a hot water bottle to bed in June, in the house... She has never complained about being cold in the TDSIII. It weighs about 1.8kg, so you could take three of them along instead of the Trakker.
My advice is to work out first what you will use the bag for, and whether it needs to go in a rucksack. Then choose a bag that will meet your needs without overkill. As a general rule, if you lay the bag out and let it loft (fluff up), the warmest bag will be the one that is thickest. Have a look in a camping shop and try with a reputable make. Price is no guarantee of warmth, neither is weight. Warmth with light weight IS possible, but costs money!
Also mummy bags may feel more restrictive, but they are definitely warmer, and if you don't use the baffles and drawcords properly, you need to add another 5-10 degrees to the rating, as all the heat escapes out of the top of the bag!
In my experience, people usually follow the following sequence:-
1) Cheap bag claiming arctic ability! COLD
2) More expensive bag claiming arctic ability from better manufacturer. Too warm, and huge pack size/weight
3) A lower rated bag from the same quality of maker. Warm enough and smaller.
4) Finally, a very expensive bag of the same rating, but a lot lighter/smaller.
It's an expensive learning curve!
