Yes the HELIX 5 DI and HELIX 5 DI-GPS have the dedicated DI sonar beam, utilizing only one element in the transducer for this. The HELIX 5 SI-GPS model unit uses the Si sonar beams to make the Di sonar that it uses.
As far as which is better and which has better picture quality… that is up to the individual. Some like the sharper images that the dedicated DI sonar can make but others enjoy the wider area of coverage, more 3D-like quality of the picture and adjustability of the Si derived Di sonar.