Many books show the building of steelstring guitars with domed tops but as I've discussed with William R. Cumpiano (Co-author of "Guitarmaking - Tradition and Technology") he also recommends building real flat tops (at least for beginners). He says that he has heard wonderful sounding guitars made with both techniques.
A guitar with a domed top will not be acoustically superior automatically. It will be stronger in the long run but if you are a beginner and if you do not shape the braces very precisely to the rather complicated dome of the top I'll bet that a well-made flat-top guitar will survive any sloppy-made domed-top guitar. Later on under the load of the strings a flat top will become convex anyway.
Anyhow, building a flat top will be a whole lot easier! Think of the shaping and gluing-on the braces and think how easy the making of the binding and purfling rabbets would be when side and top are perpenticular to each other.
Because a flat back looks "sunken in" it should be made domed.