Umm what? They both have horrific bases and bad join times, but Wendy's got good growths at least, while Sophia, well, take a look for yourself.
Hp Mag Skill Spd Lck Def Res
People think she has good growths, but man, the only passable growths here are a 55 in mag and 40 in skill, and well the ridiculous res growth is great, every mage in existence will double and severely damage her while she is weighed down by flux, or not weighed down at all. Not to mention you are more then halfway through the game, and she will be averaging 50% accuracy against most enemies. Don't use Sophia. Ever.
For units I recommend, Normal mode you can use legitimately anyone you want (except Sophia, who still sucks majorly.) On hard mode, Marcus, Zealot, Echidna, and Percival are all good prepromotes for a couple chapters after they join, and Niime is the best unit in the game for warp-skipping, and does well in combat. Units that you can use throughout the game include Lance, Alan, Shanna (I prefer her to Tate even in hard mode, the best summary of this I can give is this video: https://www.youtube.com/watch?v=WREY779phfU), Rutger, Shin (Be wary, Sacae sucks. A LOT), Miledy, Lugh, and Clairine.