DL's upgrade possibilities for Hawaii-based pax are limited to upgrading with miles, and that is further limited to fare classes that exclude the lowest priced tickets. If you check this forum, you will generally see at least two active threads about how worthless DL SkyMiles have become. DL has even been getting bad press on the value of the FF program.

The actual product DL offers to and from Hawaii is pretty good on the A330 aircraft out of MSP, DTW, and ATL. There is a 763 out of SLC, but it has domestic F seating rather than the nicer international BE seating. Otherwise, they use a lot of 757's of varying vintage.

I am happy with AA. I have yet to fail to get an award flight I can live with to the mainland at their lowest AAward level. In fact, on my last four free flights I was able to get the exact dates I wanted (in both directions) on three of them with a shift of two days on the fourth. Upgrades are limited, but at least they have them on a generally reasonable level, depending on your status. Check aa.com or the AA forum for details. I am also fond of CO's aircraft and service from HNL to and from IAH and EWR, but I don't like their FF program as much as AA.