SXJ:

i was in a similar predicament this year. i'm a silver medallion (exp 12/98), and a premier (exp 2/99). but i wasn't able to fly enough this year to requalify for BOTH of them, so to decide which one to keep, i made these personal comparisons which were most important to me:

>>availabilty from pensacola,florida
only delta flies from PNS, but i usually drive the 2 hours to new orleans to save $200-300 on ticket prices, so this wasn't that big of a decision.
small ADVANTAGE-->DELTA

>>what type of awards to i want? hawaii & round-the-world first-class
united=35,000 miles to hawaii / 360,000 RTW
delta=30,000 miles to hawaii / 310,000 RTW
ADVANTAGE-->DELTA

>>which is easier to requalify for?
i fly to europe 2-3 times a year, and delta allows requalification with 5 atlantic crossings; united doesn't (although i will concede that 3 european trips WOULD give me the 25,000 miles to requalify on united).
ADVANTAGE-->DELTA

>>money-saving perks?
delta allows silver medallions to renew its crown room membership for $275 this year minus a $50 american express coupon that i have=$225/year.
united charges $300 for a premier membership.
ADVANTAGE-->DELTA

>>upgrades?
my experiences with delta upgrades have been terrible:
1-no L-class upgrades
2-800-mile SEGMENT upgrades requires me to use 2 upgrades for a "short 330 mile" pensacola-atlanta-tampa flight; united would only require one 500-mile upgrade (if they flew from pensacola, darn it)
3-delta will not let me upgrade a companion
BIG ADVANTAGE-->united

i'd rather fly united for their upgrade policies, but i ended up choosing delta because it was easier to requalify for this year.