Akai APC 40
PROS
-Cheap! ($399 with free shipping from some vendors in the USA)
-Available in most music stores. No need to wait for months to get a new unit.
-programmable (only using Ableton MAX $$)
-Offers instant Ableton live integration
-hardware and software support from Akai

CONS
-Can only be used with Ableton.
-MIDI only controller, no OSC
-No MIDI in / out jacks
-Fixed MIDI mappings. Need to purchase Ableton Max to program the unit.
-Build quality could be low (i am sure it is built in China)
-Closed architecture. Cannot be easily hacked.
-Cheesy plastic case. (the whole thing is basically plastic, just the bottom “chassis” is metal)

Livid’s Ohm64
PROS
-Made in the USA
-Parts of the unit are open source, so it can be expanded and programmed using programs like Max / MSP / Reaktor
-includes MIDI mapping editor software
-features MIDI in and out and USB ports
-Not limited to Ableton Live use
-Can be used as a MIDI instrument.
-Free Cell DNA video software included
-More controller buttons (64) than APC40 (40)
-Better DJ-styled physical layout than the APC 40

CONS
-More Expensive than the APC 40 ($599)
-Not available yet. (Shipping June 15th 2009) Can only be purchased directly from manufacturer
-OSC support could be vaporware
-Firmware is NOT open source (as stated by Peter Nyboer)
-Body made of wood (this could be a plus for those who love furniture, but i prefer my instruments made of cold, hard metal.)

Here you can watch a few of videos showing the Livid’s Ohm64 in action with Ableton Live. Very impressive.

I think that for the money, this unit is a better competitor as an Ableton Controller to the Akai APC40 than the monome, since it offers more physical interface options.
I hope that Livid opens their firmware to the community so the instrument can be expanded by users and programmers making it a viable alternative to the APC40.

monome 64
PROS
-built in the usa (you may think this is a con if you live overseas hehe)
-community supported via open source software. Tons of apps can be downloaded for the monome and run via MAX runtime.
-programmable
-good hardware documentation: can be extended, expanded and hacked
-Made of Sturdy Wood and aluminum. Strong enough to stand on (see pic below)

CONS
-Expensive (even when purchased used)
-Scarce, It takes months of waiting to purchase brand new.

Akai APC 40
PROS
-Cheap! ($399 with free shipping from some vendors in the USA)
-Available in most music stores. No need to wait for months to get a new unit.
-programmable (only using Ableton MAX $$)
-Has more controls and offers better Ableton integration than the monome 64
-hardware and software support from Akai

CONS
-Can only be used with Ableton.
-Need to purchase Ableton Max to program the unit.
-Build quality could be low (i am sure it is built in China)
-Closed architecture. Cannot be easily hacked.
-”Chassis” is supposedly made of “metal” but the whole thing is mostly made of cheesy plastic (see pics below)

After weighing the pros and cons (specially the price) i think (in my humble opinion OK?) the Akai APC 40 is a winner just because of the price, only if you plan to using Ableton.
If you are lucky enough to get a monome kit for cheap, i think the monome is the winner.

Either way the reviews for the APC 40 are good.
Check out this video review by DJ Tom Cosm which demonstrates some of the APC 40 features.
As Tom Cosm says at the end of the review, the APC40 is going to revolutionize Live DJ / music playing as thousands of artists will be able to afford and use one.
Disclosure: I am in not being sponsored by Akai and i am in no way associated with them.