Both types are efficacious. The loss of appetite is due to the mild appetite suppressant effect of Metformin.
Some people report that tummy upsets settle down after a couple of weeks but others just cannot bear the digestive problems and are put on slow release. Still others report that even with the slow release version the gastric upset persists so they are either taken off Metformin altogether or they are taken off it and if bg has not improved they are put on stronger drugs.
Edited to add. Dietary changes are imo much more efficacious than Metformin.
I was put onto Metformin and a statin at diagnosis and was really ill - on just one tablet.
I threw the tablets away and stuck to a low carb diet. It took some time for the effects to wear off, but I do feel so much better and my BG levels when taken at random tend to be below 6mmol/l. I did get one which was almost 15 a few days ago, but I think it must have been wrong - the strips are in their last month before expiry, and my husband eats things with sugar - but I feel seriously well. I was demonstrating a Cotswold Morris jig last night....
I was put on normal metformin within 3 days my ibs flared up had a really bad attack, gp switched me to the slow release so much better very little nausea now they both work the same though so don’t worry about switching if that’s what you decide to do x