In fact, now that the IS250 is discounted, I think the TSX is a harder sell when new.
The IS option packages can push up the price really quickly. From what I recall of my drive of the TSX (long time ago!)...I thought it "felt" sportier than the IS.
Though I really like the IS, I preferred the TSX mainly because I preferred its "drive" to that of the IS. (I also liked its more spacious interior, higher seating position, and better visibility). The IS of course has other advantages and I think it's just a question of priorities.
Would I buy the new one (which I don't like esthetically as much) over the "dropped in price" IS? If it still drives the same, I'd still probably take it again over the IS.
But that's me. At similar prices, I suspect the superior cache of Lexus and the V6 will have the IS stealing market share from would-be TSX buyers.