Seriously, you have to try a 77 or you'll always wonder. I'd be really surprised if you are ever willing to give it up, it is simply one of the unique benefits of being a Pentaxian. Others may get some good portraits with some very expensive gear, but the 77 gives you a unique rendering advantage that they wish they had.
Like a previous poster said, I think you'd be much more likely to decide to sell that Tamron 90. Not that it's a bad lens, on the contrary, but it doesn't have the unique beauty to it's rendering that the 77 has.
Keep in mind that once the 77 seduces you, you are very likely to want the 31, another lens that tends to haunt you until you own it. Then, you guessed it, you get seduced all over again. This photography stuff can be scandalous!