Spruce tends to be fairly bright and mahogany more mellow. The strings can also have an effect. I have a concert uke with a solid spruce top and Aquila strings and its tone is definitely bright. (Don't know about NATO - only know the initials as a military organisation :-} )

When I bought it I also tried the Mahogany version of the same uke and it was more mellow in tone, but it was strung with different strings - possibly Worth or GHS as they were black - and that could have been a factor as well. I liked the bright tone, so I went for the spruce top.

Mine is a brand called Korala and it cost me £100 - so a step or so up from an entry level uke.

I also have a Lani soprano which cost me £60. It's all laminate so doesn't ring so well but it was well set up when I got it and the intonation is spot on - I checked up to the 12th fret in the shop and it was still in tune. I use it mostly for practising and I put it in the car when I go away.

Lani Ukes are imported to the UK by the Music Room in Cleckheaton though I got my from my local music shop. I definitely recommend them as something a little better than the brightly coloured entry level ukes, though the latter do have a certain charm. Of the these, the Makala Dolphin gets consistently good reviews.