Maybe there's a third situation - if UKIP take something like 100 seats.
That's a doomsday scenario if I ever heard one!
If UKIP take 100 seats then I would envision it making the Tories weaker than Labour, potentially opening the door for a minority Labour government to stroll into Westminster.
In this situation I couldn't see the Tories enter into a coalition with Labour because it would destroy them. I mean that literally. As in, the Tories would cease to exist.
1) 100 UKIP seats would indicate that the Tory support base has been totally fractured.
2) By joining Labour the Tories would be
by far the junior partner.
3) Being seen as "propping up Labour" would anger the remaining Tory support base. UKIP would light their smoke signals and finish the job of recruiting eurosceptics and hard right-wingers remaining in the Tory party.
4) If enough of these Tory MPs resigned, it could possibly bring down the government.
5) Snap election results in a Labour majority with UKIP having more seats than the Tories.
6) Labour absorbs the moderate elements of the remaining Tory party.
7) England becomes Scotland in being a Labour heartland, using a "it's us or UKIP" and "we can't afford
independence leaving the EU" as their only slogans.