Look into Bas Rutten his 02 trainer. Great fpr your cardio great for the lungs!!
Training Mask is a great product! I have tried both 02 trainer and Training Mask.
O2 Training causes me to drool and leak saliva out of the holes or "screens" that they gave me.
I also had an issue with rebreathing my spit
with Training Mask i had no issues breathing or anything and the product was very comfortable and fit great.
Its up to you but i dont like having to train with somthing i have to concentrate on keeping somthing in my mouth!