Yea, I probably won't run Assasin's Trophy in my Sidisi deck, for the same reasons you mentioned, but my God it's a beautiful card.
I've never used Taigam's second ability either while Underrealm's second ability may be useful in a pinch, and it being a zombie opens up the possibility of cost reduction with Rooftop Storm & Undead Warchief even though the deck produces enough mana where that generally isn't a problem.
Like you said, it will just come down to which of the first abilities people prefer.
Westvale Abbey is one of my favorite pet cards. If I ever get to run / flip it I am immediately having fun.
I love Westvale Abbey ever since it was spoiled. I run it in my Shadowborn deck as an alternative sac outlet (I run Dictate of Erebos and Gravepact and they're usually among the first things I tutor for).