The most competitive firms are very selective and will mostly target the top 3 the most. These include quant trading firms, i.e. the likes of Jane Street etc. The money you can get at these firms is a hell of a lot more than most other firms, and you could easily make up for the differential in a few years. Caveat being that there is obviously no ease in getting in.
FAANG firms (still pay very well) will be less prescriptive for unis, and will visit both unis though. Still, employers from both quant and top tech firms sponsor Imperial's Computing society very heavily, and this means that students can have more access to sponsor events in addition to the usual careers fairs that are company wide. Another thought to be had.
Imperial Computing soc sponsors:
https://docsoc.co.uk/sponsors/
Edinburgh's Computing soc sponsors:
https://comp-soc.com/partners
Also should the student wish to look for Silicon Valley/US jobs, Imperial's name will carry more cachet. This won't be important to many kids.
With all that said, past entry level, your uni name matters less and the portfolio is more important (minus the quant firms still). Also, Edinburgh has grim weather that would put me off (half-joking). Anyway, for maximising job prospects I would say Imperial. Edinburgh is still very respectable, but even with London (which has extortionate living costs), I'd take the opportunity to be in an exciting metropolis and potential to live as a student at discounted prices close-ish to campus in some of the nicest neighbourhoods in London...