Over Sapphiro, emeralde can be good for:
1. Unlimited Domestic + Intl lounges
2. No OVL / Cash withdrawal charges. (Don't use it though)
3. Forex markup of 1.5%
4. BMS 750 off on second ticket (Sapphiro has 500 off on second ticket).
IMO, don't upgrade to emeralde, try for emeralde private. Because every renewal year, they will give 12,500 points, effectively making the card fees only the GST. Also it's metal card.
However, if one can get Emeralde as LTF, it would be really good card.