I am currently using Tidal lossless and Spotify Premium side by side.
The reason for this is simply that I had a good deal for a Tidal 4 months subscription.
Eventually, one will have to go.
I would say when casually comparing the two, perhaps in unfamiliar surroundings etc, it could be rather difficult to tell the them apart.
In a familiar system of half decent quality the differences are however audible. This mostly manifests itself at the frequency extremes ... tighter, more solid bass and more air higher up if that makes sense.
Still, it is perhaps not as simple as that. Spotify (Premium) sounds more punchy, bouncy or whatever you want to call it. Some may think it is even more 'musical'.
Tidal is more hifi ... you pays your money.
Tidal has announced high resolution streaming at no additional cost to the premium service (£20/month). Whilst the majority of 'normal' (non-hifi obsessed) people will be happy with MP3 quality and would cringe at paying twenty quid per month on cloud music (when you can have it free on youtube for nothing ... as my friends daughter which is probably more representative than you and I, said ). For audiophiles however this represents probably a very good deal, especially if you have a high quality system. I probably don't count myself as having one of those but must admit I am still very tempted to keep on using it if this HR streaming takes place. - Subscribers of this premium Tidal service will probably be a tiny minority but it will set new standards in price/performance which in turn perhaps will encourage the competition to catch up ... not a bad thing ... and perhaps even drive a nail into current 'high resolution' download sites, depending on quality of course.
We'll see.
But, as has been said, Spotify has to my mind a better user interface and suggestions.