They are quite different with different rewards and challenges.
The L&C is shorter, more direct; the TransAm is quieter with less traffic.
1. I suggest the Washington side of the Columbia from Astoria.
A but longer, but beautiful and way less traffic.
The highlight of the L&C is the Columbia River Gorge - stunningly beautiful.
Plus, the Mitchell Tunnel should be complete next year.
https://stateparks.oregon.gov/index.cfm?do=park.profile&parkId=113https://www.oregon.gov/odot/Regions/PublishingImages/HistoricHighwaySegments.jpgThe drawback is that there are busy sections of US 30 and some riding on I-84 shoulders.
The long, empty stretch east of The Dalles can be monotonous.
If you do the L&C, do NOT ride the US 12 stretch in Idaho, take the Winchester option.
2. The TransAm western section has changed a bit over the years.
It used to go all the way from Astoria to Florence on the coast before heading inland.
US 101 can be pretty busy in summer - and a good deal of the time you are away from the ocean.
The one-way stretch of Old Hwy 101 is magical - but is in the bypassed section.
Depending on when you do this, McKenzie Pass may not be open and you need to use Santiam Pass.
There is a small window in the late spring when McKenzie is open to cyclists/hikers only.
Eastern Oregon is gorgeous with adequate services for cyclists.
The Snake River Canyon is HOT in late summer. Whitebird Summit is a killer.
Photos -
Biker/Hiker only stretch of the Historic Columbia Highway
Ocean view, One-lane stretch of Old Hwy 101