Based on a 20 second check, I think that a 400SM is a one of these:
http://www.motorbikestoday.com/reviews/Images/suz_drz400sm_3_lge.jpg
light weight, relatively tall-dual purpose, kind of thingy. Just want to make sure we are talking about the same bike. If, given the plethora of bike designations, you rode a completely different bike, then “never mind”.
1. The wind. Sitting upright means you get the wind, unless something redirects it. This is true for all cruisers and standards. Windshields and fairings keep wind off the rider. Some people like the wind – they learn to lean into it, or just hang on like many cruiser riders (some back support can help). Riding leaning forward on a sport bike, may feel strange in traffic, but allows the wind to actually help support you at speed. The lightness of the bike is not going to change whether you feel you are going to be blown off.
The lightness of the bike will affect how much the bike gets blown in the wind, or truck generated air flow, on a highway. So will height, a little (broader sail). Streamlining makes some difference with head winds, but bikes aren’t designed to deflect crosswinds or tailwinds. My guess is that, as a practical matter, any bike defined on this site as a beginner bike (and many others) will be affected by moderate winds at speed. I suppose that we all have to learn to deal with it.
If you are worried about being blown off the bike, you might get this feeling less on a ninja 250 or 500 that have small windshields and fairings. They also are shorter.
2) You are asking about going from a tall dirt type bike to a standard (though faired) semi-sport bike. I haven’t ridden the ninjas, but I have ridden a NIghhakw (standard, naked honda 250), and I have sat on both Ninjas. The new 250 is more upright than the old 250 or the 500 (which is unchanged from last year). I am 6′ 190 and felt more comfortable sitting on the 2008 250 than I did on the 500 or an older ninja 250. Downgrade? Depends on that you mean. The 500 will still go 0-60 faster than my M5 (at least on paper). It has both a bigger and faster engine than the Suzuki. I suspect that in terms of acceleration, the Ninja 250 is not a downgrade. But, the 400 was not designed to be a sport bike. Both ninjas are reported to handle well. But I’d be happier taking the 400 on a dirt road. At 6’2″ there is undeniable appeal about a taller bike that is light enough to handle easily. But upgrade/downgrade is apples/oranges, sort of meaningless.
3) I’m a new rider, and I’m 54. I find just sitting on a cruiser with my feet in front of me, to be uncomfortable, and a strain on my back. I’d love less bend in my knee (more distance from seat to peg) but haven’t yet found a bike I liked that wasn’t more powerful and heavy than I like. Moral: do more research and look around. Unless you know what feels comfortable, it isn’t easy to answer your questions. As others have done, I recommend “The Complete Idiot’s Guide to Motorcycles (fourth ed).
Although this is a subject where opinions differ, I am of the cc’s are not the only variable. A 650 cc cruiser is not the same as a 650 cc sport bike. 2 cylinders aren’t the same as 4. Do more homework.
Finally, you didn’t mention obtaining any formal training, and said that you had no prior experience. But you took the bike on a highway to work? Perhaps the MSF course? If nothing else, you’ll get to ride a different bike. Good luck.
You need to do some research (and I do too). The Suzuki 400 SM is a one cylinder engine producing 40bhp (I think) , and weighs in at 300 pounds dry.
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