150K BIS on UA metal will get you 60 500 mile upgrades, good for domestic (well, region 1) use which should be enough for your 6-7 domestic trips? Even if you don't upgrade all of them, you at least have E+.

You can upgrade a Y or B fare on UA for 15k miles, and a M or H fare for 30K miles. You can also upgrade Q, V & W fares with a SWU. You get 6 SWU's each year as a 1K. You will also get 8 CR-1's which can be used to confirm upgrades in region 1 on UA in advance.

Also consider that your paid Business Class trips (C or D fares) can be upgraded to international F, which doesn't even exist on NW.

I'd switch to UA.