Short answer: either will be good...
Longer answer: Dyno is parallel rail design and really WIDE OFO [one foot off] tail measurement compared with the rounder outline/narrower OFO but wider wide-point of the 3S+. I've found that the Dyno can be actively encouraged to get going in lesser winds than almost any other board of its type, and it's become my marginal wind weapon [v1 95l, 82kegs], but downsize to a smaller board as soon as I can.
Which will be 'best' likely depends on how you like to sail in waves, and the proportion of wave-time to B+J time.
Re sails: the 6.7 and 5.6 are probably the sweet spot, but that's a lot of board [whichever you choose] for a 4.9m.
For blasting with a single fin and flatwater the 8.1 could work but it's above the recommended for SV Dyno, optimistically 7.5m listed. Don't know about the 3S+ range. The 3S+ being a tiny bit longer might make it better for that, but they're both all-round freewaves with less rocker flat than say a SV Fox.
Windsurf did a recent 'review' of the Dyno 125 saying good up to 7.8 and for turning, and float and ride wave-sailing:
www.windsurf.co.uk/severne-dyno-3-125-2022-test-review/What other boards do you have in your quiver?