I hope i can help u... I have seen both quite well.
Esl is different, da passage questions asked r very direct so u can't apply dat 4 da first language
which requires to identify da genre, da author's style nd impact, questions of inferences from da passage like, "what did da writer mean by this?"
Moreover, da writing tasks for da first language r longer nd need a higher level
for ESl, words needed 4 da extended r 150-200 but for da first language u need 350-400
even da tasks needed themselves r different. E.g. in ESL, u maybe asked to write a friendly letter about ur home or a festival or something u lost nd da other would be an article about smoking.
But for da first language, write an argumentative essay about a topic like 'whether parties r da only worthwhile thing in life' nd da other would be about writin a short story where for example weather plays a significant role or ends with some quote.
So u see there is a noticeable difference nd if u make up ur mind 2 take first language u need to undergo a higher preparation!
Not much advantage simply dat it is another subject but all candidates will be
BE native eng speakers. So tougher competition...!