On price, Fly.io starts lower at $1.94/mo against $30/mo for Rocket.net. Rocket.net carries the higher rating overall, at 4.9/5, and Rocket.net reports the better tracked uptime at 99.99%. Fly.io has been operating longer, since 2017.
If entry price is what decides it, start with Fly.io. If rating and reliability matter more than the sticker price, Rocket.net has the edge. Either way, read the full profile before you buy — the trade-offs section on each page covers what the summary numbers leave out.