The verdict
If momentum matters more than pixel control, and you often need a deck, a document or a quick microsite fast, pick Gamma. Edit the draft, and read every card before it leaves the building.
If you send many sales or pitch decks under your name and need them on brand by default across a team, pick Beautiful.ai.
One practical test settles most cases. Apply your brand colours, fonts and logo in both during the trial. If Gamma bends to your brand well enough, its speed wins. If it does not, Beautiful.ai is the safer choice. If a strict brand template is the main requirement, I'd lean to Beautiful.ai.