Those of us with 400s can espouse its numerous attributes, but few of us can give you a direct comparison with the Forza, simply because it hasn't been out very long. So from my very limited experience with the Forza, here are my meager observations...
Honda makes a fine motorcycle, my experience with them is that they are practically bulletproof. However Suzuki makes a very good bike also. Quality wise, I give a very slight edge to Honda.
I have sat on the Forza, it seemed a bit small to me. The Burgman 400 is a full-size bike. (Mine actually requires a Harley full dress tourer bike cover) I have not looked at the specifications on the Forza, but it felt like the Burgman was at least 100 pounds heavier. That is important for stability riding highways. Plus an extra 100 cc engine gives this big advantage to the Suzuki. Personally, I think the 400 is one of the best smaller freeway bikes on the market. Freeway is it's forte.
The Suzuki also has the Honda in protection, ergos, storage, and the available used bikes. Hence cost also. Given the Forza is a new bike, you might want to wait a year or so till they have all the bugs worked out. This is not uncommon. Suzuki's first year of the new model 400 also had problems. Any 400, model year 2008+, is going to be a very rock solid performer. They have not changed very much in the intervening years.
I'm sure the Forza will be a great bike in the end. But it does lend itself more to city riding. So your decision really comes down to what kind of riding do you think you will be doing; Do you want a city bike that you can occasionally pop on the freeway? Or do you want a highway bike that you can ride around the city? IMHO.
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