[Brightcolours]

The problem is that for telezoom lenses there are not many high quality (build-wise) dedicated APS-C lenses on the market which is probably why Klaus selected the Pentax DA* lens. However, if you want to do fair comparisons you have to pick those pairs that match. So you could select the following pairs:

1) Pentax 50-135/F2.8 vs Canon/Nikon 70-200/F4 - same equivalent focal length and similar speed (2.8x1.53=4.28)

2) Pentax 60-250/F2.8 vs Canon/Nikon 100-400/F4 - similar equivalent focal length (90-380 for Pentax) and speed

These two pairs are also comparable on a build quality (non-budget; environmentally sealed).

 

Looking on a comparison of the Canon 70-200/F4 vs a Canon 70-300/F4-5.6, which doesn't match that nicely neither on the focal length (short end) nor on the speed, you would need to pick the "L" version from Canon to have comparable lenses. Weight: 760g vs 1050 g

 

As for missing IS, Pentax has in-body IS so it wouldn't make much sense to add this to their lenses.

 

At the end of the day it's more a question of what camera-lens(es) combo better fits to your needs and not so much FF vs APS-C vs µFT.

You can simply pick the 70-200mm f4's for APS-C. Does it matter in any way that they are "FF" lenses? No.