I've used various british army version - old OG ones, and newer OG, DPM , and MTP versions with the extra triangle of fabric at the opening - certainly makes getting in easier as the hood area is huge - essentially it opens to your waist. To be honest, i've only seen the old OG ones that don't have this triangle of fabric all the DPM and MTP versions i've come across seem to have the updated design with the wider opening. They are my choice for bushcraft as i don't tend to carry my kit very far, but when i do carry my stuff a distance, then i use an Alkpkit hunka.
As described above - the alpkits bivvis are much much lighter (less than half the weight) and more compact. I've slept on concrete with mine and not had any issues, so are fairly robust, but if strength is important to you, the issue ones will win hands down. Note that the standard size alpkit hunka is quite a tight fit - i'm tall - 6ft3, and reletively slim, and it only just fits me with a 2 season sleeping bag. If you like a bit more room, or wish to use a warmer sleeping bag without fear of the bivi bag reducing the loft, go for a Hunka XL - which are approx the same dimensions as the issue bag, but still lighter. Breathability of the alpkit versions has always been fine for me, although on paper at least they aren't supposed to be as breathable as the issue bags.