205
I just swapped my wife's 203 for a 205 in her K5 when the 203 broke. I've been abusing my 205 in my truck for years and it's never made a squeak.
Basic differences between 203 and 205 are:
203 is chain driven and weaker than the gear driven 205. I have never heard of a 205 breaking unless under extreme use.
203 is full time which causes uneeded wear on 4 wheel drive parts and drop in gas mileage. 205 is part time so it can be either a 2 wheel drive or 4 wheel drive, saving in gas and maintenance costs.
You can modify the 205 to dual stick shifting relatively easily, letting you control the front axles speed independantly from the rear axles. DO NOT EVER PUT THE FRONT AND REAR AXLES IN LOW AND HIGH AT THE SAME TIME! You'll break something bad. But you can stick one axle in neutral and one in gear and do fun stuff.
Lastly the 205 is shorter, which is good for driveline angles when lifts are concerned. So you'll need to lengthen your driveshaft about 6" when swapping to a 205. This costs around $120.
Hope this helps!