re: which resort will have the best chance of snow for the 1st week in april
Honestly this year you will probably be too late given the limited snow base, but the best piste management is in Bansko so there you will have the best chance. Last year we skied until about the middle of April but it was getting very slushy and you really only have a couple of hours in the morning before you're waterskiing. Try a higher Alpine resort.
re: which resort will have the best chance of snow for the 1st week in april
Yup agree with Mat, Borovets has a lot at the moment as well but this year has been poor. I would wait until the last minute to book to be on the safe side.