i mainly used dutch onions down to having asian stores nearby. i have used english onions too ie from asda. spanish i feel would work too. i also know some like red onion (adding an extra sweetness).
i like the dutch down to ease of peeling. i don't really think there is any discernable difference in the finished base.
the importance is how the base is cooked. there is much posted on this (chewytikka 3 hr video being the best for me). i like to have plenty of oil in the base during cooking and not much water (the onions produce water). a slow medium simmer for a long time is my preference. i also see a need for 2 stages (typ 2 hrs & 1 hr). the 2nd stage being after blending when i add lots of water.
That's the method I'm using from now on. I used to add a litre of tap water to boil 1-1.5 kg onion, ending up with me boiling away the onion's own water along with the tap water. I've just made CBM Little India home version, using 2kg onion & approx 150ml tap water, which barely registered in the pot. Very low heat for 2 hrs. Resulted in around 1ltr of syrupy onion juice being produced.As Jerry points out, the right consistency can be gained with tap water later on to suit. Thats definitely made a difference to the flavour & aroma. Not had time to cook with it yet
Regards
ELW