If all DBS/POSB credit cards get DBS points, is there a difference if I convert them to miles compared OR if will it be the same if I use the DBS points earned by DBS Altitude?

All DBS/POSB cards earn DBS Points, but conversion gbapp rates differ. DBS Altitude earns points at a rate optimized for miles conversion, so using Altitude points usually gives you better value when converting to KrisFlyer or Asia Miles.

Short answer, no.

DBS pools all the points you earn via the various credit cards you have with them into 1 pot. So when you convert the points to miles, DBS just take from that 1 pot you have, regardless which card contributed to the pot.

It is just that the points you earn from Altitude card don't expire while points from other cards expire generally within 1 year.

You can get more miles per dollar spent if you use the Altitude card for targeted categories. For Altitude, use it to buy air tickets and/or pay for hotel bookings online will give you 3 miles per dollar spent, compared to 1.2 miles per dollar spent on general spending.