Justin plays a younger version of Oliver Queen while Amell's take on the character is about 5 years his senior.
They both use bow and arrow go figure where Stephen has more longbow hunter’s gadgets comic vibe where Justin has a more standard gadgets of green arrow Justin’s green arrow compound bow can be folded into a crossbow pistol.
Both have a taste for blonde women with a techy side Justin have Chloe and Stephen has felicity. Justin married Chloe at the end of smallville while Stephen so far has only had felicity as his fiancée. Justin’s green arrow has had relationships of various natures with Tess Mercer and Lois lane. Stephen arrow has had a one night stand with Isabel Rochev who later takes on a ravager type role unlike in the comics where she is strictly a business rival. He’s also had relations with Helena Bertinelli/The Huntress, Laurel Lance, and Sarah Lance.
Stephen Amell's arrow was trained on the island of liean yu when Oliver was in his college years he was trained by the skilled archer of yeo fei and later by slade Wilson. Amell's arrow didn't lose both of his parents much later in his life when he is grown adult. Justin's green arrow lost both of his parents around the time when he was just five years old. Justin's green arrow was trained by a secret society of archers his mentor was the dark archer vordigan.
Justin's green arrow offers to train a former prostitute mia dearden aka speedy. Amell having a show centered on his character he has had 2 sidekicks arsenal/Roy harper and later on down the line Oliver aligns himself with his younger sister Thea dearden queen.