100% Savage.... I haven't owned a Rebel, but I have ridden and worked on a couple...
The Rebel is anemic at highway speeds,(low 70's mph with no headwinds)... the Savage is just minimally adequate, but,... it is adequate(88mph...at least 80mph uphill with a headwind)...
... and the Rebel has valves like toothpicks... I've seen some bent ones, and they look like you could bend them with your thumb...
It's a great beginner bike, and mpg commuter,... but, the Savage has 4 times the grunt... very capable on the highway, but not a distance highway cruiser,...
it prefers to enjoy the scenery...
...
Do a very simple "spark plug door" mod, and change the petcock to a Raptor ($30.00),... and the Savage is a maintenance dream...
If you feel you need more than that,... I'd recommend going to a 650 DP bike for maximum power with easier maintenance than your Ninja...
Look for Screw adjustable valves (not bucket and shim) and air-cooling for easiest maintenance...
Hydraulic valves would be easiest, but I don't know of an air-cooled single with hydraulic valves...
Harley Sportsters have hydraulic valves, single carb, and air-cooling, but are way overweight (200lbs more than a Savage, and top heavy) and tend to be unreliable...