Both flashes are fantastic, I own and use both of them equally. You can read my review of the
58 AF2 here for my opinions on that flash, but I'd like to point out a few advantages the 45CL has over the 58, and why I still use it.
First, the classic potato masher design is just fast and easy. The 45CL just plugs right into the PC socket of my K20D, but you can also get a SCA module to have TTL functionality and with that, it mounts on the hotshoe which may be important to the K-x/K-r crowd. The SCA system will also allow you to use the CL45 with other camera brands whereas the 58 is "dedicated" to individual brands. The 45CL doesn't require a light stand if you want the flash off camera, but I would recommend getting a
coiled sync cord, the one included with the flash is woefully short.
As was mentioned, the 45CL is actually a stronger flash than the 58 AF2, it just lacks a zoom head. For basic headshots, say in the 50~105mm range, the 58 AF2 has the GN advantage, but for lighting a whole scene at 35mm, the crown goes to the 45CL. I think the only other flashgun that can beat it is the Metz 76 MZ-5, but you could buy a whole studio kit for what it would cost you to purchase it new.
Speaking of studio lights, the 58 AF2 as of firmware 2.0 "features" preflash suppression. What that means is you can't trigger it with studio lights because they only fire one burst, and the 58 will think that's the preflash and just sit there waiting for a second burst that's not coming. You can make it work with a radio trigger, but it's just cumbersome, and depending on your radio trigger, may be costly. With the 45CL, you can buy a 10 dollar optical slave and plug it right into the sync cable and you've got another light for your studio setup.
The last thing that I really love about the 45CL is in auto mode you can "lie" to it. Say you're shooting outdoors on a sunny day at f/8 and you want to soften the shadows but not destroy them. You can set the flash to f/5.6 and it will just take your word for it, and give you a nice pleasing fill light. The 58 will do the same thing, but you have to have it set up wirelessly. If the 58 AF2 it's connected to the camera, either on the hotshoe or on a bracket via TTL cord, it knows the aperture and will just blast away at f/8. To me this kind of diminishes the usefulness of the feature, I like "dumb" auto mode better.
I will also say that if you can find a used analog version of the CL45, it works just fine, and it's cheaper. Pentax thankfully shielded their PC sockets against high voltages (31 volts is the figure I've heard, but it has never been officially released). I've been using the analog version of the CL45 for 3 years on my K20D with no problems, but I wouldn't try it with a Canon or Nikon, I hear they fry at around 8 to 12 volts.
Price wise the 58 AF2 is a better deal if you buy both flashes new, it just has more features, but you should be able to find plenty of analog and digital 45CLs on the used market for considerably less as everyone ditches them for the latest-and-greatest new thang, and as others have mentioned, they're built to last. Hope that helps.