Plenty of other ways to skin a cat. Lots of folks have used the RS amp, though it's rated slightly less than 1W and doesn't have very impressive published specs. If you plan to use a small powerhorn it should prove adequate. Not so great for larger cone speakers.
The T Amp was a more recent development than when I built my caller and subsequent TechPage piece I wrote back in 2003 (my attempt to squirm out of responding to a flood of individual inquiries). There was a lot of buzz on the internet about the T-amp a couple of or three years back, including a number of PM boosters. I seem to remember looking at the specs but couldn't suss out whether the T-amp has brick-wall 20KHz filtering or not. That and the thing weighs ~1 lb versus a couple of ounces for the Velleman. While it may be the bees knees, I haven't heard any long-term reports or success stories back from predator callers using it. I'd seen them selling for as little as $15, once upon a time. Then they got discontinued and became collectible or some such. (Last time I looked they were going for closer to a hundred bucks?)
If you go with the Velleman K4001, realize that you're just buying an electronics board with no housing that only has solder posts for hooking up your inputs and outputs. No jacks come with it.
You'll no doubt want to wire up a quick disconnect of one sort or another for your battery. Molex connectors are popular, since these are the type that 9.6 Volt DC NiCad or NiMH R/C race car packs use. Female 1/4" spade-lug wireless terminals will be the ticket if you power yours with a gel-cell intead.
I used transistor radio snap-tabs because they work with the 8-cell AA holders, only to find that standard ones didn't have substantial enough wiring. Radio Shack has HD tabs nowadays.
You'll probably also want to put a charging jack and an on-off switch in the wiring loom while you're at it. And a 1 or 2 A in line fuse is probably a good precaution too, as is a cover or project box for the amp itself, so that you don't get an electrical short from something metallic coming into contact with the bare amp board and starting a forest fire someday.
LionHo