Normally the heavier one is the warmer one if they are new. Old fibres may break like dry spaghetti though why your lighter and newer one might be as warm as the heavier old one.
Have a look at the Snugpak Special Forces system and the videos about it as well as the combination of Carinthia Defence 4 and Tropen and the British, Dutch and US military two bag sleep systems. MMS / modular sleep systems.
Like that you can compare the weight with your own sleeping bags and and estimate their recommended comfort temperature. Aging sleeping bags loose the ability to retain warmth though.
Otherwise you will understand by watching the videos that also you can put the slightly smaller sleeping bag into the slightly larger one in order to create a two bag sleep system that should serve you well down to serious winter temperatures.
3 kg of insulation are 3 kilogram of insulation and work new until approximately -20°C, used and very old surely still until -10°C both assuming that you wear a fleece or merino suit in it, as usually assumed when recommendations are given officially.
Well, and 1 kg serves around +10°C or a bit less, and 1800 g approximately around -7°C. 2 kg around -10°C and so on.
The technical differences are as big or small as the individual sensibilities and even different shapes from day to day.
Nobody can tell you how old and especially how often washed your used sleeping bags are and how much of quality they already lost or not.
You have to try out what suits you until which temperature. But that's the case with brand new equipment as well.
A gardener/ landscaper who works all year round outside will surely be 10°C less sensible for cold weather than someone who lives in a overheated office, a over heated car and a overheated apartment.
I am used to be outdoors.
A full stomach, full of salami, cheese, chocolate and a bit bread makes compared to an empty stomach for me up to 5°C or even more. Another 5°C if I am exhausted or not. Illness another 5°C.
The Ajungilak has approximately the quality of the Snugpak or a Carinthia bag or the British/ Dutch Fesca sleep system if new. The Carinthia and British made Snugpaks just last longer and work slightly better, especially in the long run.
If your Snugpak is imported that's a lower quality. But roughly said: Polyester filling is polyester filling. Weight is weight.
And you decide what's still comfortable.
One can sleep on the balcony, by the way. If it gets cold you crouch into your warm bed inside but note the temperature.
That's the most scientific you can do.
