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?

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.

Certain DBS cards earn DBS points which do not expire. So if your card earns DBS points that do not expire, you can accumulate them till you have enough to get the award flight you want. If your card has an expiry date for DBS points, then they will expire one year from the quarterly period in which they were earned.

Conversion wise both are the same. If your DBS points are about to expire, transferring them to Krisflyer or other frequent flyer programs will extend their lifespan by 3 years. You may consider saving up the points earned on Altitude for a big redemption. Altitude earns 3MPD on online flight and hotel transactions, so targeted spending on bonus categories like these can really bump up your point balance.