Do the 2004 route, most other people will be doing that so you have a better chance of finding riding companions.
Sound advice for the BCM virgin MSeries, although if you're very strong in "Welsh spring weather" the scenic can repay with some extra rewards.
A rough summing-up of the differences:-
1) 2004 - slightly faster roads, although a km or so further.
Scenic - Mainly lanes, a bit quieter, possibly not so fast.
2) 2004 - Main roads all the way to next control, slightly longer (IIRC).
Scenic - Lanes to Buith, and again through the Elan Valley, probably more work as the lumps are more pronounced.
3) Route same for both stages, until the final drop into Dolellau - when Scenic takes slightly more direct route.
4, 5) Route same for both stages.
6) Route almost same for both routes, difference being scenic climbs out of Dolgellau slightly more direct. And a bit of a lumpier short-cut mid-way through the stage.
7) The scenic has a very short divert after the first climb, and does climb-up after Knighton. but then the routes converge before Weobley
the 2004 route is slightly longer (by the odd km) but does get to enjoy the Llancloudy climb. The Scenic misses-out on Llancloudy, but finds plenty to climb in it's place. Once in Monmouth - the routes to the finish are identical.
I've managed 6 BCMs, the rides being split 3 of each.
The smart move is to enter the 2004, and do the scenic bits if you want.