That depends on a number of factors, the/my most important one perhaps being what you're planning on doing after you started building. Umbraco 11 hits its' EoL date sooner than Umbraco 10. If you're planning on building the site, handing it over to your client and have it be there for all eternity, 10 might be better. If you're planning on starting and keeping the platform up-to-date, and want to utilize the latest features & patches that come out on Umbraco, it might be worth your time to start on 11 straight away