Just be aware that all MXs in the same Dashboard Organization must have the same license level. If you start with SD-WAN+ then all subsequent MXs will also require SD-WAN+ SD-WAN+ does have some massively useful features though (Smart VPN exclusions, SD-Internet, Insight...), so it's a pretty good choice, if you're using dual Internet links at your site and rely heavily on a mixture of cloud IaaS and SaaS applications.
If you do buy Advanced Security or SD-WAN+ don't forget to enable some of the key provided features; IDS/IPS, malware protection, content filtering etc.
