There's also a Houdini Pro version that's intended for power users with high-end hardware. The Pro version supports up to 128 threads, 128 GB of hash memory, large memory pages, the use of Nalimov end game table bases and it is NUMA-aware. For most users the plain old Houdini 5.1 will be the engine of choice.
The redesigned evaluation function is supposed to take better take king safety and piece play into account which gives it a combination of an aggressive playing style plus sound positional and strategic understanding. Supposedly, when other engines see no way to make progress, or head for a draw, Houdini sometimes comes up hidden resources. The ads state that the preliminary release version of Houdini 5 won convincingly ahead of all other top programs in the recent “TCEC Season 9 Rapid” tournament, scoring 50 wins and 12 draws and the final release of Houdini 5 surpassed this performance by another 30 Elo points. I noticed this was a “rapid” tournament and some programmers tune their engines to perform well in these rapid events, but that's not what interests us. We want to know how good does the engine perform at slow setting and how does it do against Stockfish and Komodo?
On the IPON rating list games played at 5 minutes plus 3 seconds per move (mean game length is abou6 16 Minutes), Stockfish 8 (rated 3390) is still first. It's followed by Komodo 10.4 (3387) and Houdini 5.01 (3281)
On the CCRL 40/40 rating list Stockfish 8 is also rated number on at 3390. Houdini 5.01 is second at (3387) and Komodo 10.4 is third at 3383.
In head to head competition on CCLR, Stockfish holds the edge over Komodo, scoring +7 -4 =41 and against Houdini it has a more modest +14 -3 =123. Houdini 5.1 has the edge on Komodo 10.1 by a score of +8 -4 =40.
So, it appears that Stockfish 8 is still the best deal. Houdini 5.1 costs about $40 and the Pro version about $64. Komodo 10.1 costs about $60 and with the cost for the 3-5 updates per year is about $100. Stockfish 8 costs $0.00, and it seems like both Houdini and Komodo have a ways to go to catch up, especially since Stockfish is free.
Here is a game in which Stockfish defeated Houdini 5.01. Even to my amateurish eye it looks like Stockfish is still the better engine.