I see them as being very different from each other-- all you have to do is look at what gains Merrils approval/friendship in DA2 compared to what gains Solas's approval and friendship/love.. Merrill from when you first meet her in DAO is all about saving the knowledge even if it means sacrificing people- she is willing to potentially sacrifice Tamlen and the Warden out her desire to take explore the Elven ruins in the Brecilian Forest
Merrill and Mahariel are searching for Tamlen in the ruins; that's the reason Keeper Marethari sends them there. In fact, Tamlen's loss (and potentially Mahariel's) are what propels Merrill to build her own Eluvian.
later in DA2 when faced with a choice between continuing to rebuild the Eluvian at the behest of a corrupted spirit (since that is what demons are) and sacrificing her clan and the Keeper who was practically a mother to her - she willingly chooses the object over the people even if she regrets it in the end.
That's not what happened. Merrill is building the Eluvian from lore she gathered about the Eluvians, and information she extrapolated from the shard; there's no choice made between the Sabare Clan or the Eluvian. Merrill sought out Audacity because she needed to learn blood magic, but she brokered no bargain with the spirit. Merrill specifically left the clan because she wanted to go her own way, and build the Eluvian, which Marethari opposed. Marethari also kept the clan at Sundermount longer than they should have been there, as Merrill explicitly notes in Act III.
yes you can convince her not to sacrifice Maretheri and the clan if your relationship is good enough but then she gets mad at you.
Merrill is angry at Hawke in Act II because he can withhold what she earned when she invoked vir sulevanan and asked for the arulin'holm; I find it rather problematic that Hawke can basically act like he knows better than Merrill when he withholds the tool from her, because Hawke knows next to nothing about elven culture, the Dalish, or the Eluvians.
Solas in contrast is willing to put people first- moreso mages and elves than humans and non-mages yes, but he still puts them first even if it means that buildings are turned into rubble, or historical artifacts are lost... I think this would be a major bone of contention between the two of them esp what Merrill was willing to do to protect the Eluvian.
Merrill is building the Eluvian because she wants to help the People; she thinks the technology can benefit them, and "The Masked Empire" would suggest she was right about how useful it can be to the Elvhen. Solas also tries to help the People, as we discover throughout the narrative.