Both are cynical in their view of Love, but both bring it from opposite ends of the spectrum, which at the very lease makes them different in that respect.
Morrigan was raised by Flemeth in near-seclusion for most of her life, taught Love is a weakness and to shun personal attachment. She was raised to be anti-social for a reason. She was brought up to be a self-reliant lone wolf, friendship is something she never thought she wanted, or needed (until she found out about Flemeth’s plan, as she explains it.) She has little experience with interaction with others and is very socially awkward.
Isabella’s disdain for Love seems to come more from experience and personal failings with attachment, if I recall correctly it’s intimated in DA2 that she was once married, and it ended badly. She’s a social butterfly to a certain extent who gets along far better with companions (save the few who refer to her as a ******) and is less socially awkward than Morrigan, using the pretense of having fun as an excuse to not getting closer, as opposed to Morrigan who uses lack of experience and seeing no real benefit from getting close.
In short, Isabela having loved too much, and having lost too many times; contrasted by Morrigan, as having no real concept of love. Tsunderes? Yes technically both probably are, but is that any different from many other characters who start severely disliking the PC but eventually come to be BFFs, like Zevran, Shale, Nathaniel... when you first meet Wynn in the Tower she threatens to strike you down, etc. etc.
Yes, they can be viewed as similar Tsunderes romances, and the theme's of earning their trust are similar as they share similar feelings toward attachment, but polar opposites on how they came to be that way. I've enjoyed both romances, breaking down their walls, though similar, but knowing the differentiating back stories makes it different IMHO.
I find it more interesting tearing down Morrigan's walls, than sweet talking Leliana, giving her a few gifts then finding out she's in love.