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Honestly neither bike is really meant for trails... no fans, smaller radiators, no lights, close ratio gear box, stuff like that make them not ideal for trail riding. You could change all that, but then why not buy the trail version in the first place? Also with a used MX bike, you have to assume it has been ridden hard and raced, after all thats why most people buy these bikes. As for the KX450 vs RMZ450, biggest difference is the electric start on the KX450, and the last few new KX450s that I have ridden I really liked. However...

In the exact same predicament as you OP, I think I’m leaning more towards the 2023 KX. With the current rebates the dealership near me has about 10 leftover 23 KX450’s for 5500, almost got a leftover 22 RMZ for 6k but the e-start and lower price is much more appealing for me. Aside from a KLX110 I have never owned a big bike that’s not a Honda. Really disappointing that leftover 22 and 23 Hondas are still about the same price as a brand new 2024.

If you are trail riding a 450, don't worry about longevity... one advantage of the kawi is electric start when you are stuck on the side of a hill or precarious situation. Not ridden a kawi, maybe its hydro clutch is lighter and more suited to the trails also Rmz is a great bike, is it heavier than kawi?? Again, a lighter bike is good in the trails.

KX

Trails, electric starter wins hands down.

wouldnt go for a 450 mx bike if you only ride singel track style of tracks. I dont know but your trails might be wide and fast and if thats the case a 450 would work but if it is tight trails I would go for a 350. Got a KX450 -23 and would not like to ride it on our trails wich is pritty narrow and tight. Super happy with it on mx track tho. Got a KTM 450 and 350 EXC-F and they are very nice on enduro/trails/singel track or whatever u call it. Much smoother and easier...

Kawi a no brainier for me

It doesn’t matter other than price, just get the best deal. Any 450 you get will be great! All the bikes are reliable., I don’t even think that’s an issue. I’ve had Yamaha, Suzuki, Honda, husqavarna all in the last 5 years all 450s and haven’t had an issue with any of them. I ride 4-5 days a week about 3 hrs at the track . Pick the bike you like at a price you can afford.

I have a 08 250f with 1000 hrs on it. It's literally the loaner bike. Still kicking with basic maintenance. Any 450 you get is gonna be fine. Get either the best priced bike or get the bike with the most for what you want to spend. If your mostly trail riding the button is nice if there's a good chance of you stalling it in odd positions.

Current KX has won or come in second in most shootouts for the past 4 years. While shootouts aren’t everything, it’s still impressive. I’ve not heard of reliability issues beyond the chain guard. Plus it’s lighter and has estart. Seems like a no brainer, particularly with the deals you can get now on them.

Look for a KX450x.

Why a 2019? 2023 Kawis are dirt cheap at dealers from the rebates they have going on. Suzuki your still getting the same bike, but I saw a dealer around here have a 2023 450 for like 5800 OTD. For me if i want reliability its definitely buying new. That way I know the bike hasn't been messed with.