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I think your best bet is to decide exaclty what you want, and get exactly that. If you want to go with iron sights, get one with a fixed carry handle. If you want to go with a scope/optics, go for a flat top. You can scope a fixed carry handle rifle and you can mount iron sights on a flat top, but neither is likely to give you optimal results.
The barrel length is much the same. If you're looking for something for use in tight quarters and shortish range, go for a carbine. If you're looking to do longer range shooting or going for the smallest groups possible, go for the full length. Again, you can work around either, but your results likely won't be optimal.
That said, if you're just looking for something to plink with at the range and don't envision going out past 100m, any quality AR (and the bushmaster more than fits the bill) will suffice. I'd go with a fixed carry handle. It's the simplest and cheapest option, be it carbine or rifle. No screws to come loose, easy to sight in, and fewer things to break.
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I have all the parts, and assembly will commence this afternoon. (I don't have the stock spanner, but I know someone who does)
