Decat question

I tuck off my decat today and is in bad condition so changing it.I have a spare one here but has a small opening at turbo side.
Would it be ok to fit it and what be the advantages/disadvantages of fitting the smaller one instead of the bigger one?
Here a pic of the 2

Go on to youtube theres a video on the different types of decat by mrt just type that in and try look for it

Thanks. I'll try find that