Hello Kimberley. Millie's lovely - she positively smiles, in her picture.
It's always tricky trying to decide what to do when a situation like this arises, and I think your worry about her running off is one which plagues every one of us as a cat owner at some point or other when there's a move in the offing.
I think from the information you've provided, Milly would probably be happier with you. They do sometimes miss their old territories if they're largely outdoor cats, but as Gillian says, if you keep her indoors at first and let her explore and investigate in her own time, she should settle in well. Be aware of her environment, and assess potential risks and needs such as access and proximity to main roads, etc. For instance if she's been used to having freedom to come and go, but won't at the new house, you'll have to think abut whether she'll have shelter if you're out and she can't get in, that sort of thing.
Cats can be very adaptable.
I determined when we moved that I'd keep Paddy in for around 4 weeks, and then only let him out in my company, although the first thing I did was fit a catflap, as our house didn't have one when we moved in.
Paddy had other ideas, and after the first few days was desperate to be out - we'd moved from a house near a busy metro line, and with only a tiny backyard - and then there he was with a front and back garden - and a field - and us stinkers were keeping him prisoner! Anyway, after a week, I reluctantly gave in and let him out. He had a good wander round and decided it would do him just fine, but I honestly wouldn't recommend letting Millie out that early unless you're fairly confident she could cope. Paddy was a little older than Milly when we moved, but he settled down well. Having said that, thankfully he's never been one to roam far - the same could not be said of my first cat!
Let us know what you decide, and I hope the moves goes nice and smoothly for all of you.
