If you are still 1K you can match to DL Plat for the year, which is a good matching level because it includes C+ at booking (Gold does not, which might surprise you coming from UA). In general I've found DL to be a better airline domestically (better OTP, newer aircraft, happier and friendlier crew) but their international network is somewhat limited and expensive and SkyTeam doesn't hold a candle to Star Alliance. For now, their qualification is on the now-old system where distance counts, so it may be easier or harder for you depending on your travel patterns. In general, DL lounges blow the United Clubs out of the water
However, schedule should still play a big role. You say none of the majors dominate in AUS, but if your travel is to SFO, ORD, or IAD, maybe your choice of carrier still matters.