Originally posted by Igilligan I should say I have 'an' answer.
And it is something I now wish I had done. I stuggled between the K20 upgrade or the D90 with its better AF and Flash system + the bonus Shallow DOF video... but like you I had money in lenses and I love the inexpensive old glass you can get for the pentax... and other than the known limits of the K100d I really enjoy the camera, so I ended up with the K20.
I now wish I had done it differently... I really like the K20 and sensor wise it is a real upgrade to the K100d (cropping ability is amazing)... but the issues I had with the K100 are still there with the K20... The ability to track a moving kid is still not up to the nikon. The Pttl flash system is... well inconsistant at best.
What I wish I would have done is to keep my K100d for my schnaportraits shots of the kids and for using all the old glass in the ways I love using it. Then commited to the Nikon D90 for what I want it to do... action shots of the kids and consistent flash shots . Plus those really fun "hollywood" shallow DOF video shots.
My kids are just going to get bigger and faster... I should have commited to that concept. And kept the pentax for the other side of this hobby.
That is what I would advise you to attempt if at all possible.
good luck
Igilligan's reply is bang on what I went through in my crazy "try this, buy that, no wait, sell, and buy again" search for the "right" system to settle into. Here is the analogy I used in my (hopefully) final solution:
Choosing a camera or "system" to shoot with is like buying a gun in regards to what you are going to shoot. IE: You ain't gonna take a .30-06 to go duck hunting and a Remington 870 Wingmaster is far from the best choice for deer hunting. Either one can be used and under the right circumstances will work. But lets face it, you will be better off buying the right tool for the job.
If you want to "shoot" at both, you simply must use 2 different products to have the best chance at nailing the target. If you can't afford both, either live with the shortcomings, or shell (bad pun intended) out for the right equipment.
So how does my violent analogy translate to my decision to ditch the D90 for the K20D? While I have not worked out my final write up on that here is a quick summary from pictures I shot at the wedding (as a distant assistant) last weekend:
1. Detail: No contest, the K20D with a Sigma 24-70 f/2.8 lens wins over the D90 coupled to a 18-105mm VR lens.
2. Focus lock: The Nikon walks away from the K20D. Under bad lighting it is no contest and even under great lighting the D90 is noticeably faster.
3. Metering: This is a really tough call. The D90/SB-600 flash is nothing short of phenomenal when it comes to nailing exposure. The K20D/AF-540 does a decent job, but it can be "fooled" by wide contrast variations in a scene that the D90 just takes in stride. That being said, the more I shoot with the K20D, the better my understanding I have of what works best for exposure when composing a shot.
4. Battery life: The D90 can get over 750 shots and still have gas in the tank while the K20D is sputtering before it gets to 600. This nothing more than acknowledgment of the facts. In reality, you are going to have spare batteries so just learn what the limits of power are and have the proper amount of batteries to accommodate your session.
Bottom line: The K20D just flat out takes a better picture under the right circumstances. The D90 will let you get away with more when it comes to speed shooting. But don't expect the same level of detail, color and contrast. And before the Nikon fans start shredding my results by comparing the "kit" lens the D90 was using compared to the Sigma f/2.8, understand I NEED image stabilization with my less than steady hands. And since the Nikon system offers NOTHING in the f/2.8 zoom range I wanted, well there you go.
If I was made of money I would have a D300 for speed, A D700 for detail, and a Fuji S5 Pro for dynamic range that would let you over expose a shot of a bride wrestling with a polar bear on a iceberg at high noon and STILL be able to pull the detail back in with PP. And of course the thousands of dollars of lenses to mount on the bodies.
But since I am not made of money I will shoot with (and enjoy) the K20D and live within the parameters of the system.
Rick
PS: Once I get the time to PP my images I will get some examples posted. Right now the primary photographer has all my memory cards.
Oh, by the way the primary photographer was shooting with a Canon 5D Mk II with a host of "L" glass. I would get into that but this post is long enough already. But it is a "Scha-weet" system. With a price tag that made me shudder.