You have to clearly understand what you'll be gaining and what you'll giving up while switching to K5.
K5 - the "+":
-size
-much newer sensor with better both low-iso and high-iso, bigger resolution and awesome dynamic range.
-in-body SR
-whisper-quite shutter
-probably better out-of-the-camera JPG as D300s is really intended for the RAW-users
-LTD primes (the smallest primes for any APS-C and FF systems)
-"over 30 years of K and m42 glass"
-better auto-iso
*I'm not including WR in the "+"es as it's of any importance only if you stick to WR'ed lenses, otherwise it's just a fetish-gimmick for Pentaxians to go orgasmic about.
K5 - the "-":
-stone-aged AF. Yes K5 is a huge improvement over the previous Pentax bodies, but it's still light-years behind the D300s/7D. In all departments: accuracy, consistency, # of AF points, tracking). D90 actually has a better AF than K5.
-very limited choice of lenses and for the most part, quality Pentax glass is more expensive. Used market is almost non-existent.
-if you're planing to use the flash in any other modes than directly in-da-face, full-manual is the only way to be on the same wavelength with the camera. Same goes for wireless.
-high pixel density sensor (same goes for the Nikon D7000), which implies big RAW-files, which implies that if you're shooting a lot and processing a lot - you gonna question your need for extra 4MPs a lot, as it does slow things down.
-finniky QC.
-Dodgy SDM in DA* lenses
D300s the "+":
-AF
-FLASH that just works!!! no questions asked!!!
-12mp sensor that produces considerably smaller files.
-huge used market
-huge choice of lenses
-over 30 years of F-mount glass.
-better compatibility with mass-consumer PP-software (RAW processing)
-all pentax DA* glass available from Tokina for the fraction of what Pentax costs, without the WR that is, and without SDM troubles.
-overall handling of the camera that is very robust, professional, dependent and consistent. Pentax gear always has some degree of "toy" feeling to it.
D300s the "-":
-damn bulky!!!
-loud shutter
-dated sensor
-noisy low-iso
-only in-lens VR
-finniky AUTO-ISO that only suits prime lenses and still requires some manual input.
If i were you, i'd consider trading the D300s for the D7000 or D90 instead. In my experience D90 is still the most "photographer's" camera for the money paid. It actually has a more accurate AF than D7000 (nevermind K5) as well as sensor and flash-capabilities of D300/D300s. Very capable and balances (feature-wise) camera body. D7000 is great as well, though I, personally, prefer the output from D300s and D90.
Last edited by alexeyga; 02-27-2012 at 09:37 AM.