MEC - Helium 600

I don't really get the hype around alpkit and think some of their gear is overrated. If you want a reliably warm bag then you can't go wrong with a mountain equipment helium. I've got a helium 600 large and it's an excellent bag

The Spark series of bags is ultralight, so makes some compromises to achieve that. The fabric has a significantly lower denier rating than the Neutrino (10D shell and 7D lining versus 20D for both in the Rab) so may be more prone to snagging and other damage, and also has a lighter weight zip. The Spark does have a slightly higher fill power (850 versus 800) so will be slightly warmer for its weight, which is quite a bit lighter than the Rab. So ultimately it comes down to whether you’re prioritising lower weight over long term durability. Both are high quality bags, just with slightly different priorities. Be aware that Rab bags have an alpine fit which is quite narrow. If I may suggest another option to consider: the Mountain Equipment Helium range, specifically the 400 or 600. I use the 600 throughout the winter and for its weight (mine is 950g), it's extremely warm. The Mountain Equipment bags are also very comfortably sized, a bit more roomy than the Rabs.

Glad to be of help! I actually considered getting a Spark bag myself a few years back and did a load of research, ultimately deciding against them because I’m one of those people that’s quite hard on my kit (or maybe just clumsy… 😅). I’ve been super happy with my Mountain Equipment one although you really can’t go wrong with Rab either 🙂 By the way, if you’re not familiar with UOG, I’d highly recommend checking out their site. They test a lot of the gear themselves and have lots of useful resources (explanations about things like down fill power, thermal ratings, etc): https://ultralightoutdoorgear.co.uk/sleeping-bags-and-accessories/ https://ultralightoutdoorgear.co.uk/blog/the-ultimate-guide-to-lightweight-sleeping-bags/

I also use a Helium 600 in mid winter and it’s an excellent bag.

Personally I use my Mountain Equipment Helium 600 bag for mid winter camping. But this is considerably more expensive than the ones you're looking at. If it doesn't gross you out, you could consider looking on eBay and FB Marketplace for used gear. This is the most affordable way to get higher spec outdoors gear. The Mountain Equipment Helium 600/800, the Rab Neutrino 600, Rab Ascend 800, Nemo Riff 15F would all be potentially affordable down options. Also, if you decide to save up and invest in one new, generally the manufacturers' websites are more expensive; try sites like Trekitt, Tiso, UOG, etc. They're almost always cheaper, sometimes significantly so (especially with Trekitt).