When you ride the monorail to Epcot, you will go through security twice, regardless of where you board the monorail. The first screening happens where you board (Poly or TTC), and the second happens at Epcot between the monorail exit and the park entrance. Sadly, there is no way to avoid it right now. ETA — The monorail switchpoint at the TTC is inside the security bubble and you do not need to be rescreened there.
We generally walk to the TTC to catch the Epcot loop, knowing that we may wait longer at bag check if we arrive right after the parking lot tram unloads and sends a crowd to bag check. But rarely does the extra wait exceed the time it takes to ride the monorail from the Poly to the TTC, then transfer to (and wait for) the Epcot monorail there. If you arrive between parking lot trams, the bag check line is short to non-existent and it is definitely quicker to board there.
That said, if we were already at the GCH, or were staying on the farther side of the resort, the resort monorail might be a better option. I really don’t think there is a huge difference, it just depends on whether you prefer to ride or walk. It’s great to have the option! Have fun!