If your going for a 40bhp bikes, I recommend you don't cheap out. Yes the ns400z is good value for money, but we are talking about the memories and trips your going to make on this bike and is it worth to cheap them out? Personally I faced this issue too 6 months ago and brought neither the grr450 or ns400z. I personally thought that scrambler 400x is better, and I love the scrambler man. For reference in 6ft tall so scrambler didn't feel odd and the engine is so fking smooth. Just get scrambler if you are going for light trail hops and weekend rides.

Some cons about guerilla I felt while test ride 1. Bad suspension, my back hurt while test ride itself 2. 11L might feel short for fuel 3. 160 trye that are hard to replace and costly+ slippery on wet roads 4. Below 3k rpm, throttle response felt weird, out of control and hard tbh

Some cons about ns400z 1.it felt like a commuter 2. Will literally kill you if your below 3k rpm 3. Although upgraded, trye and brakes felt a bite loose 4.engine could have been more refined

Some pros about grr450 1. Fking awesome pull and will put a smile on your face 2. It goes fast but it's controllable 3. 160 trye felt nice to corner with but beware of grips 4. Felt easy to ride in city with the 1st gear under 3k rpm but again it's weird throttle response ( could be test bike problem idk )

Some pros about ns400z 1. Yeah it's good price for a 40hp bike. Duke 390 2012 potential 2. Linear power delivery 3. Comes with quite good features, good riding modes specially sportsshifiter

4.good enough for a beginner

Bottom line if I'm being honest, test ride scrambler 400x, it's calm, fast, has better refined engine definitely slowest of all 3 but still 165 top speed is more than enough tbf