Love Lab is about a student council at a girls' school that secretly dedicates itself on the side to the art of pursuing boys. Genshiken Nidaime harks the return of a college circle dedicated to the artistic pursuits of anime, manga, and video games. Although I thoroughly enjoyed the first anime adaptation of the Genshiken manga when it aired in 2004, I did not say much about it at the time. Since then, the series spawned a number of OVA and a second television season, making Genshiken Nidaime the third or fourth season rather than the second part, depending on how you count it. (A quirk the anime itself acknowledges.) Love Lab is newer on the scene; it's the first anime adaptation of an ongoing manga (Renai Lab) that has run since 2006.
I'm finding Love Lab tough to sell, but easy to enjoy. This is mostly because the series doesn't sound like much from its description, but the execution is excellent. The jokes are funny and the characters are likable. One aspect I haven't quite gotten my head around is casting Numakura Manami as Riko. I associate her voice so much with her character Hibiki from iDOLM@STER that it's hard to accept her as anyone else. The best solution, obviously, would be to re-work Riko's character design to actually look like Hibiki so I could pretend Numakura Manami is voicing Ganaha Hibiki the 765PRO idol in a starring role portraying Riko in a production of a Love Lab television show broadcast within the iDOLM@STER universe. (I'm only partly kidding.)
Speaking of seiyuu, Genshiken Nidaime replaced basically every voice actor from the previous anime adaptations. I could sort of understand if the intent was to replace them with brand new talent, but as that doesn't appear to be the case at all, I find the change rather perplexing. It's also quite jarring to hear completely different voices from what I'm accustomed to, even though it's been a few years since I last watched any Genshiken. In particular, replacing Kawasumi Ayako with Yukana has been quite difficult to accept even though I like Yukana quite a bit. Madarame's new deeper voice also takes some getting used to.
Some fans of the original Genshiken anime expressed disappointment with Nidaime because the Genshiken club itself has changed. Previously a boys' club of openly geeky males, it has transformed over the years to one dominated by fujoshi members and their pursuit of male-on-male erotica while the alumni and their interests are presented as more incidental to to the main plot. Naturally, this is because the old members are no longer students, thus no longer part of the club, and likewise occupied by other "grown-up" interests as their original fervor wanes.
Nevertheless, Genshiken Nidaime appears to be just as much about personal growth and acceptance as it has always been. The first season focused on Sasahara as he grew more comfortable with embracing his otaku side, and on Saki as she discovered she could still love one otaku in particular (Kousaka) while still loathing otaku in general. Later, Genshiken focused on Ogiue as she came to came to grips with her identity and the parts of her life that previously mortified her. Although I have not read ahead in the manga, it appears Genshiken Nidaime will focus on the cross-dressing Hato as he deals with the challenges of being a straight male (allegedly) who masturbates to yaoi (i.e., 2D gay porn).
Notably, it appears this season will also spotlight Madarame. This former club president (now three terms removed from the current head, Ogiue), was previously depicted as the savvy, veteran otaku who was completely comfortable with being who he was. However, he clearly still carries a torch for the Genshiken Best Girl, Saki (previously Yukino Satsuki, now sorely missed despite Satou Rina's talents), and he refuses to publicly acknowledge this most heinous of perversions: Attraction to a three-dimensional woman. In this regard, perhaps Madarame has the most growing up to do.
Through five episodes, Love Lab is better than Genshiken Nidaime, but I suspect that the latter will be better remembered years later if both shows continue to progress as they have. I suspect Genshiken resonates more with aging male anime fans, even though this season is mostly comprised of female characters. Incidentally, if you know anything about Genshiken, you should already know Ogiue Maniax is its leading resource 'round these parts.