318i vs 325i

Looking to purchase a 1997 or 1998 300 series convertible, what are the different merits and problem between the enginges and the vehicles?

I dont think there was a 325i on offer in either 97/98, at least not in Europe. You may be referring to the 328i. In either case, the 325i and 328i obviously have a lot more power and torque and are therefore much quicker but not as economical as the 4 pot (318i)