Based in HNL, last year I was mid-tier on both UA (1P) and DL (GM). This year, I retained 1P and am PM on DL thanks to those double base miles promos.

DL UPGRADES
I always purchase at least a K fare on DL, so my domestic upgrade success rate to the west coast and beyond as a GM using segment upgrades has been 99% over the past 7 years. Never had to purchase any SMUs because they are generous. DL's NAMUs came in handy last year, especially when I had to connect though ATL or CVG. I sure miss the SWUs but being a PM softens the blow somewhat.

DL doesn't allow international upgrades at the gate, so that is a sore spot, however, I have been fortunate with SWUs and now PMUs and have had them clear between booking and a month prior to departure. DL international upgrade success rate as a GM and now PM - 100%.

UA UPGRADES
UA lets you upgrade domestic flights on just about any fare. My domestic upgrade success rate to the west coast has been around 60% using segment upgrades. I did purchase a set of 4 500-milers when I first became a 1P. On expiration, they are credited to your account at the rate of 500 miles per. 1Ps cannot earn HK49s (NAMU equivalent) or receive comp SWUs - these benefits are reserved exclusively for 1Ks.

The bulk of my UA travel is international. To fly to Asia on DL metal, I would have to fly to ATL to get to NRT. HNL-ATL is equivalent to HNL-ICN. If I am in a hurry to get to Europe, I will fly UA. UA provides one-stop connections to Europe from the West Coast. DL metal requires two connections to get to Europe from HNL. And of course, it's difficult to beat UA's 744 service from HNL-NRT, especially in seat 15A on the upper deck. UA's international upgrade policies are great, albeit I have to burn miles. Mine have cleared between time of booking (usually 2 mos. out) and at the gate. UA international upgrade success rate as a 1P - 100%.

MILEAGE ACCRUAL
DL GMs receive 100% status miles and 100% elite bonus miles when flying DL and all SkyTeam partners. UA 1Ps receive the same on UA metal only as well as LH flights between Germany and the U.S. Otherwise, 1Ps and 1Ks for that matter, will have to settle for 100% status miles and no elite bonus miles when traveling on Star Alliance partners. Some Star partners will give you reduced status mileage (70%) if you fly on discounted Y fares.

LOUNGE ACCESS
On UA, 1P status gets you Star Alliance Gold status and gives you access to all RCCs and international lounges abroad with a same day international ticket. GM status means nothing with regard to lounge access. You have to attain PM status to get the same lounge access on SkyTeam partners.

All in all, DL is my primary carrier to the mainland and business/leisure trips to Europe. UA is my secondary carrier for business trips to Europe and primary carrier to Asia.