Mike,
Go to a specialty art store and check out the 'shaper' brushes. Here's a pic of what I'm talking about:
I use the one that's second from the left most often. It's round, but has the end cut at an angle with a slight concave to it. I also have the one that's second from the right too, and it's good for getting into corners. The tip is rubber and it works really well for spreading the putty out (almost like putting on paint).
I have some various 'putty' tools like the ones in the second picture you posted, but I find that I use these 'shaper' brushes the most.
I just happen to be putting together a 1/72 T-38 (F-5) and the fit of the various pieces is absolutely terrible. It also had raised lines which I've sanded off and will re-scribe. I have used this 'shaper' brush and putty A LOT in the last few days...
Cheers!!
Murray