Its only personal perference Im sure, but I find Lowrance to be the better brand. Ive owned Humminbird, Garmin, and Lowrance over many years and they are all pretty much dependable units, but the proof is in what you use today, and both my boats have Lowrance graph/gps units.
My Garmin display froze up on a Great Lakes trip,(Black River Bay on Lake Ontario), and considering it was a two week trip was a bit more than annoying. In fairness, Garmin repaired it, and it was flawless afterward.
Humminbird was a decent unit, however, if you use the speedometer for slow trolling you will not be happy. Humminbird replaced the unit twice, but they all were useless for that purpose.
Lowrance simply does what I want it to do without fail, every time, for three years, without quirks. I bought a second unit,(smaller model) for my jon boat, and the only thing is I wish I had gotten the five inch screen on the second one as well. The unit works as intended, just think a five inch,(or larger) screen is perferable.
Hope you find this useful.