The Galaxy S3 mini is a far better phone all round than the Galaxy Ace.
On the Ace the screen is smaller and of less quality, it runs an older version of Android, the cpu is worse and so on.
If you want a better phone go for he Galaxy S3 Mini, if you want to save money go for the Ace.
I got a Galaxy 3 Mini a week or so back on a £10.50 monthly contract with Mobile (but I had to pay £90 for the phone).
This worked out cheaper over 2 years than going for the free phone but having to pay about £22 a month.