We recently (2weeks ago) travelled out on Standard Premier as the upgrade was less than £20 each and back Standard as the upgrade was around £40 each. As with everything it depends on cost. I think the salient point is on the return journey we wished we had sucked up the extra cost of standard premier. Seats are wider and you get more space on SP, and you can also pick your seats, so we always go for a bay of 2 with a table facing each other that way you play footsie with you wife or SO rather than a stranger.... The food is a bonus but isnt really the reason for upgrading on its own. I am a big lad, 6ft, 20st and no one would describe my wife as petite and the table of 4 we were sat at on the way back had us two sat with two other bigger people, and it was cramped.
Recent experience with Eurostar is that the trains are full, both standard and SP were full going both ways, so you are not going to be able to spread out in standard, every seat will probably be taken. I get the feeling that the airport problems have been a bonus for Eurostar