On price, DreamHost starts lower at $2.59/mo against $34/mo for Pantheon. Pantheon carries the higher rating overall, at 4.6/5, and Pantheon reports the better tracked uptime at 99.99%. DreamHost has been operating longer, since 1997.
If entry price is what decides it, start with DreamHost. If rating and reliability matter more than the sticker price, Pantheon 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.