Personally, I don't see a separation between either. Dahlin plays the more important position, but that position comes with higher risk. Svechnikov plays the less important position but is probably more of a sure thing to be an impact player. The other notable difference, rarely discussed, is that forwards on average are more likely to significantly outperform their ELC's. For example, Hedman may be the better player than Tavares, but early on Tavares probably gave more value-per-dollar spent which has to be acknowledged in the cap world. Same is probably true with Stamkos vs Doughty, but it's much closer.
If I somehow had the first overall pick with my team, I'd obviously take Dahlin due to need, but it is unlikely that happens. If you are Vegas or a team like the Canucks that currently lack elite scoring player (Boeser could change this outlook within a year), you have to seriously consider Svechnikov. He looks to be a top-tier wing prospect, along with the likes of Hall, Laine, Kane, and Marner.
There are very strong arguments for either at the top of the draft. If they consider along this path, it should be a very interesting debate. I don't think there is a consensus for Dahlin right now, it seems quite split.