The main difference is that Brownies will only have girls
. As a Guide leader I am biased, but I think that is great. Perhaps not so key at Brownie age, but at Guides it starts to be nice to have a girl-only space. Another difference is the age groups - they're not quite the same: Brownies are 7-10 years, Guides 10-14 years, whereas Cubs are 8-10.5 and Scouts are 10.5-14 (I believe - hopefully a Scouter will be along to confirm that).
There is also the meeting times/dates to consider - what is more convenient to you - and waiting lists. I can't speak for the Scout Association, but in many cases Brownie and Guide units operate waiting lists and there may not be a space at your local unit if you're looking for an immediate start.
Plus, Brownie units will vary a little, as will Cub units - they have a programme framework to follow, but all leaders have different skills and, in Brownies/Guides at least, they will take input from the girls themselves, so units may have a bias toward outdoor activities, sporty stuff, craft etc. All should do a bit of everything though.
Your best bet would be to ask around, and visit if you can, to see what appeals to you and your DD. You can find out where and when your local Brownie units meet through the website: enquiryym.girlguiding.org.uk/
I hope this helps.