If you're flying a lot for work, you may well want to focus on just one airline program and its partners. Many of us find that the benefits of "elite status" become as or more important than the "free trips". You've got about 25000 RDM ("redeemable miles") for a free ticket but it sounds like some of that was from bonuses or credit cards or other non-flying.
Elite status begins at 25,000 EQM ("Elite Qualifying Miles") or as NW's new owner Delta calls them, MQM ("Medallion Qualifying Miles"). 25K flown EQM on UA gets you "Premier" (known as "2P" on FlyerTalk because that's the internal UA code for it on their ancient computers.) 25K flown EQM on NW gets you WorldPerks Silver Elite, which next year will become Delta SkyMiles Silver Medallion when Delta merges the NW WorldPerks program into Delta SkyMiles.
Status benefits vary somewhat between the programs, but here's what's in common.
1. Priority Boarding. At the Silver/Premier/25K level you don't get first priority boarding but you do get boarding ahead of non-elites in coach. This could mean the difference between overhead bin space for your rollaboard or having to check a bag for a business trip.
2. Waived checked bag fees for 1st and 2nd bag.
3. 25% bonus RDM earned when flying. Fly 1000 mile trip, you get 1000 EQM towards requalifying as elite but you get 1250 RDM towards your next awards.
4. Participation at the lowest priority level in the airline's free upgrade scheme. This differs greatly between UA and NW/DL, but both do offer upgrades to elites.
5. Priority check-in lines. An official SkyTeam benefit for NW/DL. Not an official *A benefit but offered by UA and some of its partners to *S (Star Silver 25K elites)
If you're flying 25K miles/year and you split it between UA and NW/DL, you're "dirt" on both airlines and both alliances. If you concentrate on earning elite status on just one airline and its partners, you get the benefits. If you're flying at least 50K/year, opinions will vary as to whether you should go for Silver/25K/Premier level in two programs or go for Gold/Premier Executive(1P) in just one.
If you choose NW, you're actually choosing Delta. You can earn EQM/MQM domestically on NW, DL, AS(Alaska - not in SkyTeam but a partner), and until CO leaves SkyTeam on 10/24/2009, on Continental. Plus their international partners. If you become a Silver Elite/Medallion from flying 25K, you'll get your 25% RDM bonus on almost all of their partners. You'll be eligible for "unlimited free upgrades" on domestic (North America) DL and NW flights, but you'll be at the bottom of the priority list below Gold and Platinum members. As an NW Silver for several years, I probably had about 25-40% upgrade success. If you join NW's program and earn NW Silver rather than crediting DL's program and earn DL Silver, you'll also be eligible for upgrades on CO until 10/24/2009 or sometime earlier, but definitely not after. DL Elites do NOT get upgrades on CO.
If you choose UA, you can earn UA EQM on UA, US, and shortly after 10/24/2009, on CO. If you become a 25K Premier (2P), you'll get your 25% RDM bonus on UA flights, on US flights, and on USA-Europe only Lufthansa flights, but not on any other UA partners. You will NOT get "unlimited free upgrades" but you will earn 4 "e500" Electronic Upgrade coupons good for upgrading 500 miles worth of UA only flights, for every 10,000 BIS ("Butt-in-Seat") miles you fly on United/United Express/Ted aircraft - nothing on partners counts for BIS upgrade earnings. Which means you at most will earn 8-e500 upgrades as a 25K/year flyer, which is enough to attempt to upgrade one transcon one-way and one shorter flight one-way. But you'll be at the bottom of the upgrade list behind GS (Global Services), 1K (100,000/year), 1P (50K) flyers.
For domestic reward redemption, I feel strongly that UA has far better Saver 25K RDM availability than either DL or NW (or for that matter CO or US - only AA is as good with domestic, own-metal award availability). NW and DL have relatively poor availability at their 25K level and now have a 3-tier level where often you have to pay 40K miles. Top tier is 60K miles domestic which is crazy.
For upgrades, I feel strongly that NW/DL is far better for the 25K EQM or even the 50K EQM/year flyer, in that you don't need to use "certificates" of which at those levels you earn relatively few.
Haven't gotten into the arcana of international partners, international upgrade availability, website preferences, search tools etc. But focusing just on domestic, I'd say UA if domestic awards are more important than upgrades, and NW/DL (with the NW program for 2009 until it becomes DL SM) if upgrades are more important than domestic saver availability.
I'd say splitting 25K EQM between them is a foolish waste of earning elite benefits.