Speed wobbles, and innately learning how to correct them at his age, are a feature
of a runbike
Some of them can get a wobbly, especially after the rubber that acts as a steering stop wears in (out?) a little. My two have had them a little, but I wouldn't say they'd been a problem.
You're not the only one to come to the same conclusion re: the older child struggling with the training wheels vs the younger child with good balance on the runbike. But yeah, it's hard to be scientific about it, as there's so many other factors at play.
Thing2 is baulking at her [cough] new pedalless pedalbike "I'll be big enough soon Daddy" (ie: she knows she's big enough now, but is a bit anxious about it - and fair enough, it's quite a bit bigger) so we tried a different tack on Sunday. I got Thing1 back on it and they hit the BMX track, so she could increase her confidence by following her big sister around. Worked a treat (and Thing1 enjoyed it too).
Oh, and FWIW, when Thing1 first got it, the first change made before she rode it was this.
She kept riding her runbike for a good year afterwards too - until she had completely grown too tall, as it was easier to get around on (hills/gutters etc) than her pedal bike. The runbike was for commuting, the pedal bike was for 'going for a ride'. But yeah, try paint (perhaps HE could do the painting?) or stickers work a treat.