Both fields are in very high demand and both will command decent salaries- 35-45 k range for rad tech and probably more for sonographers.
As long as you are not incompetent, there is excellent job security for both fields. Rad Techs are the ones that perform radiologic examinations, whether Plain films, CT, MR, Angiography etc. Sonographers perform ultrasounds which tends to be more operator dependent and relies more on the the hand-eye abilities than radiography. The other thing is that at least half of ultrasounds concern women's health issues. Not surprisingly there are many more women in the field than men. Another thing to consider is that there are far fewer sonographers, so you may have to take overnight calls for emergent scans from the ER, which may factor into the higher salary figures compared to RTs.
For more information, you should try posting the same questions in the appropriate forum at the Auntminnie website.