It depends. If you travel mainly domestic US, I would stick with Delta for the ease of upgrades. For me, after 20 yr a Delta guy, my upgrade success was >80% on domestic flights. I switched my business this year to United because I have a lot of travel to China/Japan. Delta promised more routes, but have cancelled DTW-KIX, and they limited service to PVG. There is also no non-stop service to PEK. But the biggest reason for my switch was the dramatic increase in Delta M-upgradeable fares from about $1700 (in 2009) to over $4,500 (current); and even with the high M-fares, there is no availability! Just this last week, without any status on United, I purchased a $1,029 RT fare DAY-PVG in April with 1 connection through ORD that was upgraded immediately to business for a co-pay of $1,200 and 60k miles..., so $2,229 and 60 k (United) compared to $4,500 and 60k (Delta), with the latter never available!
In terms Int'l service in business class, I also had a bad run of the disgruntled, elderly, Delta FAs, especially the ex-NWA ex-union ones. I'm hoping it won't be as bad on the United long haul flights. Thank God for the service and FAs on the China domestic routes
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