Start low. You can always upgrade later (Xero makes this very easy), but downgrading usually requires calling support. Pick the plan that covers your basics today, and upgrade only when you hit a wall.
Prices shown are for US customers. Xero prices vary significantly by country (UK, Australia, Canada, etc.). xero subscription prices
If you are on the Standard plan but need to track time spent on a client job to bill them accurately, you might need to upgrade to Premium just for the "Projects" feature, which can be a steep price jump just for time tracking. Start low
Xero offers various discounts and promotions to attract new customers and retain existing ones, including: xero subscription prices