Neither are outstanding, both quite low spec (£1000 doens’t buy you what it used to), though the mongoose has the better if pushed but only just. I’d still ‘maybe’ choose the GT over the Mongoose, from a personnal misplaced ‘brand’ loyalty – I’ve just never fancied/liked (a) Mongoose.
It’s worth bearing in mind that the C2W scheme isn’t exactly the ‘deal’ it used to be, and maybe 0% finance from a normal bike shop on a slightly better model (brand) might be better option for more choice & variation.
I definitely wouldn’t rush to buy one of these, thinking you getting a C2W bargain.