New lane configuration at I-84's west end

[xonhulu]

I get up to Portland quite often, and have always wondered about one thing with this split.  Maybe a Portlander can chime in.

If I'm on I-84 WB, and I want to get to US 26 WB, which is quicker?

I-5 NORTH to 405 and over to 26?

I-5 SOUTH to 405 and over to 26?

If they ever continued I-84 west along US 26, I wonder how they'd route it?

Under most circumstances, south takes considerably less time and travel than north. 

But NE2 is right, you do have to at least move left to the middle lane on I-5 in anticipation of the I-5/I-405 split on the west side of the Marquam Bridge, then move 1 or 2 lanes right on I-405 for US 26's exit.  You have plenty of time to make both lane shifts, so that's not a big barrier.

[Bickendan]

It's really a mood-dependent thing.
If you're going for the view, take the Fremont bridge. The southern route takes you on the bottom deck of the Marquam Bridge.
If you're going for time, and traffic's not an issue, take the southern route.
If you're stuck in traffic, it's a tough call:
Southbound, you have to merge to the left lanes of I-5 on the Marquam Bridge to get to I-405 north... then weave through traffic merging from SW 6th Ave, easily one of the worst weave spots in the city.
Northbound, you have to weave from the auxiliary lane onto I-5/US 30 then back through the next auxiliary lane past N Broadway to stay on US 30 (I-405 south). Then you have the traffic merging from US 30 east, NW Glisan/Everett Streets, and stuck in slower traffic getting onto the Sunset.

And finally, to your final question: It'd make more sense to send I-84 toward Lincoln City via I-5, OR 99W and OR 18 than it would toward Seaside via US 26.