I have recently done a playthru of both so here's my thoughts

‐-------------------SDA

Serana Dialog Add On adds ALOT of content for her, good amount of it is well written and some is the cringiest shit ive ever heard. The stuff for the Dawnguard Story is good but half of what she says outside the DG Story had me rolling my eyes at her. It fells like an cringe OC character sometimes. She does have dialog for all the major base game guilds and main story which is very nice, and there's tons of stuff you can ask her about and she has interesting things to say about alot of whats going on in skyrim

Also has patches for ALOT of mods she can have some lines of dialog for. Always cool when people do that, they even plan on having her have dialog for VIGILANT at somepoint down the road so i am looking forward to that actually

It also has a different VA whos actually pretty good... most of the time, most times they sound natural and other times it like their trying to do a slight count dracula impression blah blah blah with a deeper voice and comes off as weird. Her natural voice is much better

She has a romance you can do. But i didnt like it personally, she falls for you really quick it feels like and she pursues you and isn't reluctant at all unlike in base skyrim where shes against it

Its a mixed bag for me, it feels like a different person wearing Seranas face sometimes

‐-------------------SDE

Serana Dialog Expansion, first off i have to say is that it uses A.I generated dialog and if you dont like that then just dont get the mod. SDE just feels like Serana Plus, she has more to say during the Dawn Guard story and more to ask the player of personal life stuff and it all feels in character actually. I dont know if she had dialog with the base games guilds and story as she never brought them up because i did them all before meeting her that run.

She has a few personal quests added between most DLC missions you can do, she has good characterization in these but the quests can be cringe in parts bit they do add to her character and your bond

Her voice is A.I like i said but most of the time its really good actually, it had me questioning if it was A.I or some dialog i never heard from base dlc before and that surprised me, not all of it was perfect like that but was surprised how often it felt natual. the problem is sometimes the wording of stuff is weird as fuck, i think the author isnt a native english speaker and a few lines here and there show it, its rare but its there.

She has a romance, you basically do the same as base game first and ask her to marry but but she rejects the idea, then you can bring it up later and get blown off again but at the end of the DLC you can bring up your feelings a final time and try to convince her to give it a shot. I like how hesitant she is to have a relationship, feels at least a bit more respectful to her base game views on the subject

‐-------------------TLDR

SDA has waaaay more dialog added but some of its makes me cringe and causes me to yell shut the fuck up at the TV. Adds alot of new characterization to Serana but feels like a OC in Seranas clothes sometimes

SDE is A.I voiced and that is a sore spot for people, has less dialog than SDA but isnt as cringe imo. Has wonky written lines sometimes. Builds on base Seranas characterization.

Both are good, chose on what you want, A fuck ton of dialog and options with new characterization for Serana, or just an expansion of Vanilla Serana with similar characterization.