It's not necessarily a given that 75F will give "off flavors". "Estery" doesn't equate with "off flavors".
S-05, being a derivative of clean (meaning not estery) American ale yeast, is often seen as leaving little in the way of yeast character. Nottingham, being a derivative of often estery (fruity) English ale strains, is more likely to have an estery character.
Experience has shown, however, than when fermented at the lower end of its temperature range Nottingham ferments with little in the way of fruity esters. Indeed, it is touted as "neutral" - Danstar tells us Nottingham "produces low concentrations of fruity and estery aromas and has been described as neutral for ale yeast, allowing the full natural flavor of malt & hops to develop."*
S-05 "produces well balanced beers with low diacetyl and a very clean, crisp end palate."**
75F is outside the upper end of Nottingham's optimum range. 75F is listed as the extreme upper end of S-05's range. No matter which strain is chosen, at this temperature excessive ester generation is pretty much guaranteed.
I suggest you choose a mid-range temperature, like between 65 and 68F. That should permit a decent side-by-side comparison. Don't forget to taste blind.
Bob
*
http://www.danstaryeast.com/nottingham.html
**
http://www.fermentis.com/FO/pdf/HB/EN/Safale_US-05_HB.pdf