At first glance- sure they look similar, but if you look closely it's really not hard to tell the difference.The easiest way to tell the difference is probably in the coloration of their hind wings.
T. Angustipennises have a pretty transparent, slightly tanned set of hind wings, compared to
T.Sinensises who have black scattered patterns with spots of purple dabbed in the center.
Also the coloration of the body:
T. Angustipennises are usually mono-colored: If they are green, they will be mostly green through out their body without highlights or tips or gradations of color. (same with brown), as opposed to
T. Sinensises who might have (in case of the common green variation) gradations of yellow in the inward side of their raptorial arms, and also the inside of their thorax/ lower pronotum.
The T.Sinensises are usually about a tad bit larger than the T.Angustipennises too. I have seen many many T.Sinensises Much larger than T.Angustipennises but never vice-versa. (They, therefor seem to be much stronger to me too, although that might just be out of my limited experience)
Their ooths are another easy way to differentiate the species clearly:
T.Sinensis ooths are large and altogether rounder,
as opposed to T. Angustipennis ooths which are slender and smaller in shape.
The T.Sinensis ooths basically look like T.Angustipennis ooths with a whole lot of Baking powder mixed on to it- They are puffed up and T.Angustipennis ooths don't have that foamy puff
Hope that helps. They are both pretty cute though- and entertaining.
Minkyu