Two Towers boasts a meticulously-staged extended action sequence with the Battle of Helm's Deep, a clever mix of practical and digital tools to create the towering Ents and the game-changing motion-capture monster that is Gollum.
The Desolation of Smaug continues to raise the bar for immersive 3D filmmaking, builds on the Rings movie foundation to create incredibly expressive and tangible mo-cap characters/CGI visuals, and offers moviegoers the mother of all dragons (no, not that one) in the marvelously digitally-rendered Smaug.
So... which film is the more impressive, effects-wise? Well, although certain aspects of Two Towers are now somewhat dated, there is a grisliness to its production design that is sometimes lacking, yet desirable, in the polished miss-en-scene of The Desolation of Smaug. Then again, the heightened realism of CGI elements in the second Hobbit installment stands above that in the Rings movies now, even though Two Towers still blends old-school/digital age techniques rather well.
When it comes to action, The Desolation of Smaug has two spectacular sequences/set pieces (the escape from Mirkwood and journey into the Kingdom Under the Mountain), which blends sophisticated camerawork with CGI to create the impression of high-octane battles - where elves swoop like hawks, orcs thunder on the ground like wildebeests and a massive dragon coils and maneuvers like a slippery snake.
Two Towers, on the other hand, has the Battle of Helm's Deep - gruesome and mind-boggling large in scale, remaining one of the most (if not the most) grand battles of the silver screen. The eye candy might not be as dazzling as you remember in Two Towers (compared to The Desolation of Smaug, anyway), but the action choreography and sheer numbers involved with the Rings installment are still hard to top, even today.
Thus, the final decision:
Winner: The Two Towers for action, The Desolation of Smaug for effects
Both Two Towers and The Desolation of Smaug are cinematic circus performances, serving up hefty helpings of narrative bridge work, personal character moments, huge fights, big-budget qualities (costumes, sets, effects, etc.) and deeply-entrenched themes (morality, environmental concerns, spirituality, etc.) that are bubbling just beneath the surface - resonating all the stronger because the films often don't call attention to them in a fashion that prioritizes sermonizing over storytelling.
Peter Jackson's direction on Two Towers feels more confident than on Fellowship of the Ring, no doubt in part because he'd gained much experience and wisdom from shooting the first installment in the Rings trilogy - having hit the ground running on his first tour of the world of blockbusters epics. That sense of self-assurance remains abundantly clear and noticeable in The Desolation of Smaug; no matter how over-filled and bloated the film may feel at times, there's always a sense that Jackson has full control of the steering wheel now.
But does one film rule them all (see what I did there?), when it comes to this category? It's difficult to firmly declare one as being more steadily-captained than the other (for this writer, anyway), seeing how Jackson the director manages to very much accomplish what he intended with both the second Hobbit and Rings installment. The story might well be different when it comes to Return of the King versus the final Hobbit film, There and Back Again, but until then:
Winner: It's a Tie!
And So, the Overall Winner Is...