To drive future XFELs, normal-conducting linacs at various rf freqencies are being considered. With optimized accelerator structures and rf systems, a higher rf frequency linac has several advantages, such as high acceleration gradient and high rf-to-beam efficiency. This paper presents a comparison of possible S-band, C-band and X-band linac designs for two cases, single bunch operation and multibunch operation, where the bunch train length is longer than the structure fill time and the beam loading is small. General scaling laws for the main linac parameters, which can be useful in the design such linacs, are derived. Show Partial Abstract