It's really up to you. 7.3 adds a bunch of new features.
So do you have any reason to use 7.3 right now? Are there features you need?
Here is the release notes: Web Gateway 7.3 Release Notes - https://kc.mcafee.com/corporate/index?page=content&id=PD24039
7.2.0.x has been rock solid. 18.104.22.168 was just released yesterday which included a few bug fixes (for an already stable version).
Since I will start from scratch and at some point, if we start with 7.2.x, we will have to upgrade to 7.3.x, the only reason I could see to not use 7.3 would be because there is a big bug with it that make it not usable in production.
Then I would say 7.3 should be fine to move forward with.
Features will be released with each 7.3 (7.3.1, 7.3.2) branch as what was done with 7.2.
7.1.5 was the "first" release of 7.2, and 7.1.6 was the "second" release of 7.2.
See here for more details:
If you have an existing 7.2 cluster it may be a good idea to go with 7.2. If this is a separate cluster then 7.3 is fine.
We have a failover pair (active-active using Proxy.pac to load-balance/failover base on the source IP of the clients) of 1900C running 6.9.3. We will build a pair of 1 x 5000 and 1 x 5500 using 7.3