First, I don't currently own either Lowrance or Hbird side/down imaging. From following info and posts on a number of boards I do believe that Lowrance has a very slight edge in the crispness of the picture for downscan only, not side. Hbird had consistently been very competitive in quality, performance and price, so I do anticipate something new from Hbird. I wish they would announce it.
As far as the pixel count goes, HD and all that I don't see any difference. Even when the technical specs are different, you still have to be careful in how much you make out of it. The 2D sonar is a good example. On their regular units L advertises 20 degree cone angle at -3db and 60 degree fish detection. Hbird says 20 degree at -10 db. From a technical perspective it appears that the Lowrance transducer would out perform the Hbird. I have not found that to be true on the water. In fact, just the opposite. This summer I plan to do some objective, replicable on the water measurements of cone angles just to satisfy myself as to the performance compared to the specs and marketing.
Someone may do this with down/side imaging. If they do with both brands having a loyal following I'm sure it will be controversial. I won't spend any more with Lowrance because of customer service and quality concerns. I have a Camry and Tundra. May not buy Toyota again either. Everybody understands that but some don't get it if you apply the same logic to Lowrance.