i was looking at this sigma 120-400, IQ looks to be top notch on that lens, all the pics i saw taken with that lens appear to be extremely sharp and looked like the kind of IQ i would really like to get. i am not 100% sure if 400mm is long enough but i would assume it would be a pretty nice step up over the 200mm, its about the same price as the tamron 150-600
i was out bird shooting today, i took around 400 photos, all scrapped except for 15, i just could not get close enough and if i were to crop the images i took they needed so much crop just to get a good view of the bird the images no longer looked good.
A few pieces of advice:
1) If you spend less now, and you have reasonably high expectations, you will likely be left wanting more. For most sane individuals, there should be a 'good enough' performance point... the problem is that most people set their budget at a bracket which does not meet their "good enough" standards. If you're new and it's your first purchase, you'll quickly find out that you cannot know what you will be happy with since you don't have that experience yet.
2) 400mm is nice, but 600mm is nicer. If you're doing dedicated bird shooting and/or similar types of shooting, you will quickly learn that more reach is a good thing. Not only will you look to the sharpness of the lens, but you'll look at the limitations of the lens and realize that 400mm stops at 400mm. This isn't an issue with super sharp prime lenses, they can be cropped out... but a 120-400mm, you don't get as much leeway to crop.
3) Resale value is important on a lens. If you're new, there's a higher chance you'll somehow end up selling a lens soon after acquiring it. It just happens with beginners... certainly happened with me with a couple of my lenses just a year ago. Think about which lenses are in demand, and how easily you could sell an oddball lens that is a bit older.
4) Don't be afraid to spend a little more. A lens shouldn't be seen as an investment... it's not. Rather, it's an asset. You want your assets to retain value, and your assets that do retain value can be sold whenever you want to cash in on the value of that asset. If I buy a brand-new well reviewed lens for $1400 today (and let's say that's its fair market value), I can turn around and sell that lens for 75%+ what I paid in 3-4 years from now potentially. Some people can even get more than 75%, and some people can wait longer than 3-4 years before they sell at that high return rate.
If you're not in the market for the lens immediately, wait to see what Sigma drops on the market. They've officially announced two different 150-600mm lenses. If you're in the market for a lens right now, and you're on a budget, I just don't see how you can go wrong with a Tamron 150-600. The performance at 600mm is still very good, a step up from your 18-200mm at 200mm in terms of sharpness for sure (remember if you view an image on something like Pixelpeeper.com, only pay attention to shots at 1/1000 of a second or quicker, otherwise you can't be even remotely certain of whether lack of sharpness is due to the lens or the user).
I personally want to purchase the Tamron 150-600mm, I just don't have the cash available. I own the Nikon 70-300 VR.
Do note if you get a birding lens, you'll likely want to have a tripod or a monopod.