HTTPS based sites normally add a little stress to the server for handling requests, make sure you are hosted on a good performance web host for users & search engines to access content fast. If you're making HTTPS as the default version and if bots take time to crawl your website, it might after your crawl rate (and possibly ranking)
Just like how we see WWW and non-WWW, HTTP and HTTPS are looked as two different sites by search engines, I'd highly suggest you to go with 301 (permanent) re-direct from the start to save link juice. Keep your SSL certifications valid. If you don't intend to do 301 from start, check your logs often to see if you get any links to HTTP version (as most often people try to link here) and request webmasters to update the link to HTTPS if you want to see your HTTPS version ranking in SERPs.
My personal suggestion is to stick with one and doing 301 before you launch the site live.
Cheers
