The primery difference is licence, standard EULA is “free” but free in quotation mark because you will pay for it eventually if you use UE4 commercially, as you are obligated to pay royalties 5% gross revenue from your product or service that use UE4, if your product or service don’t make any money then UE4 is 100% free for you. There also series of exceptions as stated in EULA:
However, no royalty is owed on the following forms of revenue:
1. The first $3,000.00 in gross revenue for each Product per calendar quarter;
2. The first $5,000,000.00 in gross revenue for each Product from the Oculus Store;
3. Consulting fees or work-for-hire fees which are non-recoupable for services performed using the Licensed Technology (e.g., an architect-created walkthrough simulation or a contractor-developed in-house training simulator);
4. Revenue from an Unrestricted Product, including for clarity, revenue from a Product which solely relies on the Licensed Technology for production of non-interactive linear media (e.g., broadcast or streamed video files, cartoons, or movies) and which is Distributed in a form that does not contain the Licensed Technology or, in order to deliver, rely on servers running the Licensed Technology;
5. Revenue from a Product which is only Distributed to Engine Licensees (such as through the Marketplace);
6. Revenue from ancillary products which are not software and which do not contain embedded information (such as QR codes) which affects the operation of the Product (e.g., comic books, soundtracks, apparel);
7. Financial winnings generated by awards for the Product;
8. Revenue from donations for a Product which are not tied to Product access or in-Product benefits;
9. Revenue from interactive amusement park rides which use the Licensed Technology; and
10. Revenue collected from an end-user buyer of in-game items or other in-game content for your Product which is sold by an end-user seller and which is actually paid to the end-user seller and is not retained by you or any other party.
Read EULA here there is clearly stated what you can and what you can not do and what are your obligations:
Unreal Studio (aka Enterprise) have different licence, you pay monthly but you don’t pay royalties. There also extra features, so far it is only Data Smith which is content bringing plugin :
Extra templates and extra material packs. But there possibility that new exclusive features to show up.
There might be also some other licence differences optimized for “enterprice” outside of typical game development, for details read it up:
Other then that it is the same engine and it really not much difference other then licence, so since you hobbist that starts adventure with UE4, it a lot better to start with standard version which is free in daily use, licence not much effect you until you start to distribute your product anyway and since both version are compatible (again they practically the same) to each other you can always switch.