Ok, thanks everyone. The sieving times for 1281979*2^n+1 suddenly increased strongly from about 5 h at n = 578 to 9 h for n = 590. In terms of time that places the SNFS174 as a GNFS135 while the SNFS177 was already like a GNFS139.
Rational side sieving is still faster, the switch to vbcurtis's c135 parameters helped a bit, but not significantly.
Would it make sense to test the number of required relations? For c135 it's 71,000,000 and for c130 only 46,000,000. Or do SNFS and GNFS behave similar in that regard?
