From the standpoint of getting high-quality video and audio out of a camcorder, either HDMI or HD-SDI will work, but HDMI is a consumer connection--the connector doesn't lock, and it's designed for casual use. SDI, on the other hand, is a professional connection--the connectors lock, and it's designed for much heavier usage than HDMI. There have been converters that input HDMI and output HD-SDI and vice versa, but they've tended to be big and line-powered--not something that you'd want to carry with you wherever you go.
At IBC, Atomos, the Australian video recorder folks, introduced a new product, Connect, that solves the problems with existing converters. The Connect is available in two versions: HDMI to HD-SDI, and HD-SDI to HDMI. Each one is priced at 249 Euros/$349 (U.S.), and is only 42.5mm W x 29 mm H by 72.5mm L. They're powered by a Sony battery and fit on Sony, Canon and Panasonic battery plates, so they can be mounted easily on almost any device. They can also be ganged together, so that multiple converters can share a single external battery.
Atomos' Connect converters can add a HD-SDI interface to any camcorder, make DSLRs work with professional video recorders and switchers, and enable lower-cost HDMI monitors to be used with HD-SDI devices. The company expects to begin shipping the devices worldwide by the end of 2011.