According to "The Complete Book of Food Counts" by Corrine T. Netzer:

1/2 cup of turnips, boiled & cubed
3.8 gms carbs - 1.6 gms fiber = 2.2 gms carbs

1/2 cup of potato, boiled without skin
15.6 gms carbs - 1.4 gms fiber = 14.2 gms carbs

Turnips are versatile, too. There's a few interesting recipes involving turnips in the Kitchen: Low-Carb Recipes section of this forum.

-G.