To be honest, unless you've had a decent pair of headphones then you'd probably be happy with either. They don't sound miles apart from each other - the main difference between them being that the 990s are open and the 770s are closed. If you want to complicate things further than look at the 880s because they are semi open lol
I have an MMX 300 headset which is basically a pair of 32Ohm 770s with a nice mic bolted on and I also have a pair of old DT 990s from the 90s (haha!) and I think they both sound awesome.
Ultimately, unless you need the insulative properties of the 770s then I would stick with the 880s and 990s. Most people tend to prefer the airy feeling of open headphones. If you've ever used in-ear earphones then you'll know the feeling.
You might struggle to find these for around £100 though unless you're buying second hand. There's nothing wrong with buying these second hand normally as they are definitely built to last and you'll have spare parts available for a good many years to come! Just need to find a decent set somewhere hehe