way better! Sony MDs record in a compressed format (ATRAC), which compromises sound quality. Further, its at 16bit. With the R-1, you will be able to record at a higher bit rate (24bits) in an uncompressed format (.wav), and the analog to digital converters are of better quality than the MDs.
The R-1 would blow away an MD. The jury is still out as to which of the other recorders you mentioned (Marantz, MT, or R1) would be the best, but really, they all have advantages depending on the situation.