what the split means...

the number of people coming to Miami during WMC week has always been much bigger than the attendancy of the conference.
They come for all the events aside WMC :
Ultra Music Festival,
Hotel pool parties,
hotel roof parties,
Clubs,
Private parties,
free or fee all these events were guaranteed to have  amazing crowds and DJs.

But Ultra develop more and more exclusivity contract with DJ, making organizing side event more complicated, as the DJ had to have a different show than what they offered at UMF.
It is probably for that reason that WMC decided to move earlier in the months.
Despite those exclusivity deals, DJs managed to have their events one way or another, by playing with some else or playing alone if they usually play together...so this reason might not be very valid.

Anyway, what will happen is that some DJs already planned their tour around their set at Ultra and scheduled their private event the same week, while other will be willing to change their date.
Until now, nobody has yet clarified what they will do.

My guess is that  their will be enough party for you to have fun no matter when you go.

If you love outdoor festival, giant crowds, costumes, glow sticks and teens... Go to Ultra.
You will never have access to so many dj for such a price.
All DJs seem to love playing it. They have a special connection with the crowd and play different set than to their own events.
Djs on Ultra set list will probably play somewhere else too this week, either a club or a pool. So if you love the feeling of miami clubs, I think this will be the week too.

If you are a professional and want to built your network, discover new talents then you should go during WMC.
If you like to party in a slightly smaller gathering, by the side of a pool, you should go during WMC week too.
I would assume that most free pool parties will happen during WMC week because they usually compete with Ultra as they are at the same time.
The most famous one being the beatport pool party, my bet would be that wether beatport  follow WMC  move or not would be an indicator of the success of WMC move.