Both frames, Spartan in the PI and Trojan in the US, are cast. Yes there is some more machine work on the Trojan, flat top slide, undercut trigger guard and the chain link stippling on the front strap, but does that make the frame better?
My gun doc said all the internal parts with exception of guide rod and barrel were STI. So it shares a Ti hammer strut with the Trojan.
Bottom line, spend your money on what you want but be objective. If the frame modifications are worth 300 dollars then go for it. For a lot of new shooters the Spartan is a GREAT way to get into shooting SS or USPSA in general and I think it's misleading to claim that the inherent qualities of the Trojan will make it last longer, as both are nearly the same weapon just with different levels of external finish.