The way I'd look at it is this...
You can probably charge at least £20 for a full body spray tan using a decent product (Nouvatan would get my vote too). Once you've mastered spray tanning, it shouldn't take you long at all to spray a body - say 15 minutes tops - so on a busy day, you'd be able to fit in say 4 spray tans an hour, bringing you in a gross income of £80 per hour. And if you get your clients hooked on their spray tan, then you may be in the situation where your most loyal spray tan clients visit you for a tan every week or so.
Whereas for lash extensions, you might be charging, say, £70 for a full set of lashes - which would probably take, say, 2 hours - in which case you're down to £35 per hour doing lashes, compared with the £80 per hour when you're maxed out doing spray tans. So even when you're only at half capacity doing spray tans, you'd still be earning slightly more per hour than when you're doing lashes, as well as having half an hour to chill out within that hour too (or earn extra doing other treatments).
From what I've seen as well, applying lash extensions requires intense concentration and painstaking precision - so I'm imagining that most people would find it "harder" work than applying a spray tan.
In terms of appointment book flexibility too... for a full set of lashes, you'd probably need to block out at least 2 hours for an appointment - whereas for a spray tan, you'd probably only need to block out 15 minutes - so a spray tan is going to be something that's much easier to fit in at short notice, for example.
Lash extensions are longer lasting than spray tans too, so there will likely be longer gaps between appointments per client for lash extensions compared with spray tanning (assuming your client becomes a regular at either treatment).