I wouldn't pick either one!
Speer BT's is the reason I switched to flat base a long time ago. But with other manufacturers' bullets BT's work out fine. For some reason I get better accuracy with FB over BT's in Hornady bullets in general.
If I were to go with Speer I'd choose their hot core bullets (I.E - FB). If I were to choose Sierras I would stick with paper at the speeds from a 270. However, if you kept your bullets from hitting the shoulder, the Sierras 'may' work. They have failed for me.
If FB Speer bullets are not to your liking, try Interlock Hornady's.
My first choice for a non-premium bullet for the 270 is Hornady Interlock and I'd try FB and BT's or and I don't have experience with Hornady Interbond bullets, but I'd think they would be good bullets too.
I have used mono-lithic bullets in the 270 and I think that if you wanted a bullet for anything a 270 is good for the monolithic bullets work well.
Having said that if there were only Nosler Partitions available for the 270 - they might be tied, but they can't be beat.
