I started with Lucius then bought Sonnia. I just recently got Hoffman. The thing I would say about Lucius with Sonnia is that you can have Sonnia as the master and basically just bring Lucius' box set as the crew giving you an awesome synergy side (from Lucius, who doesn't really need soul stones) while keeping the full power of Sonnia (who, as you know, loves to eat soul stones.) As much as I love Hoffman... and really, I am loving playing him, his box comes in at a whopping 17 stones. So, unless you plan on playing exclusively 25 stone games, you will be left wanting more, like a $30 Peacekeeper. (I assume Mrs. Breachbarfer wouldn't be too happy about that purchase *grins*) It is also important to note that while, yes, you can bring Samael and Witchling Stalkers with Hoffman, he gains no real benefit from it like he would bringing more constructs.
All in all if you want to add new feel and new models I would say go for Lucius. If you think you can convince the Mrs. to allow a little more than a box set (or if one of your friends gets you a gift card for a present), get Hoffman. And either way... see if you can sneak a Watcher in there... because seriously, they are pretty nifty.