I have a Noco GB40 and a GBX155 which I think is their smallest and largest one. I've also had a Jump N Carry 770 for about 6-7 years.
The GBX155 was a black friday deal 18 months ago. I thought it might replace my dependable but aging JNC 770. It doesn't. I've had so many problems with the GBX that I've threatened to give it away several times. When it works, it has a huge kick - turns over even large diesel engines - but only for a few seconds and only if several other criteria are met - you can't have a completely dead battery or no battery. But when it works it will get you going.
So on prime day this year, I bought another JumpNCarry 770. It's big, bulky but it works so well, I can't live without one. I also bought several of the noco small lithium batteries - I really like them - I might even try replacing the battery pack in my old JNC770 with a lithium Noco. .
I've also had the GB40 for several months now, I leave it in my SUV under the seat, plugged into the back seat cigarette lighter, so it is always charged. It seems to be much more reliable than the GBX155. I've used it dozens of times in that period around home, in fact, it's so convenient, that I almost always go for it over the GBX155 or the JNC770 if it can do the job.
Edit: After I wrote this I took my old JNC 770 apart. It was from 2016, so 8 years old now and the battery will no longer take a charge. I tried to install a NOCO lithium battery - it didn't fit and had different specs anyway, however I found there are replacement batteries available on Amazon. The Clore one is $115 - which with tax would be 80% of the cost of the full jump pack - that seems too high. There is a Mighty Max for $59.99 which I ordered.
So I have the new JNC770 which I will carry in my truck and the old one I'll leave in my shop. The NOCO GBX155 will also stay in my truck for now, but I don't know how long I'll keep it. I definitely like the form factor and weight of the NOCOs, but I do not care for the operation of the GBX155 - I would say I can count on it only about 50% of the time whereas the JNC770 works every time.
If I were to make a recommendation on small jump packs I would say go with the cheaper GB40.