Mainly it's the length if anything will be noticed, and I think they're high prices for a 125 and a 180.
My 125 cost me £190 complete set-up and cabinet, and my Rio300 cost me £286 complete set-up and cabinet.
One rule is to buy big not too big when you first come into fish keeping and a 180 is the perfect starter size, as you've already started with a Rekord 60, you gone down a step. Just mention how nice it'll look. Tell her the price from Juwel.co.uk for the Rio180 complete set-up with cabinet aswell, I;m sure that'll make her consider it more.
Also say if you get the 125 and not the 180 you may have the craving to buy the 180 at a later date, and they might've gone back up in price.