I have BLU WIN HD LTE with dual sim.
Should I place my primary number as SIM1 or it doesn't matter?
It might be easier for you to remember it putting your primary SIM in slot 1 but it does not have any consequences functionality-wise.
Yes, there is a difference. Only the first SIM slot supports LTE.
EDIT: I was using this device as SS device. When I used my SIM in first slot I had LTE and when using in second slot it was 3G only. I remember that I've checked max speed in setting and it was LTE for first and 3G for second slot. There are maybe different versions for Europe/North America/ Asia. P.S. This Indian website also states that second slot is 3G only: gadgets.ndtv.com/blu-win-hd-lte-2858