i dont really have any track time in either car.
I am basing my comments on the condition of my e30 325 when i got it and the kind of work it needs in order to be comparable to a decent e36 328i.
I found my e36 much better at high speed corners and S turns while my e30 slides out much sooner. also, my particular e30 is a 325e so its slow as molasses. both having stock suspension. the stock suspension in almost any e30 will be worn out or atleast spongy by now while a 98 e36 has better suspension and less worn components.. of course that all depends on the car you get, but on average that should be about right. I have seen rusted out POS e36 and super clean stock e30's, but thats not average. especially here in canada.
So, the cost of a coil over kit is, lets say $1000.00
M50 or M52 can be sourced with tranny for $1000.00 (local wreckers, may or may not be in good working order)
you still need to upgrade some stuff and change some stuff for the swap at lets argue $600-$1000.00
and the E30 its self can range from $500 to $2000.00 depending on a whole wack of crap.
So, at the end of the day, you would have a really decent e30. but you have to put in alot of work (if your able, and if your not then its going to cost you 3 times as much for the swap, at a shop)
if you took that same $ amount you could go purchase a pretty decent e36 and jump on the track.
there is always exceptions to the rule and if you look hard enough you might be able to find the right car, but i am arguing the law of averages, or atleast what is most common in my area.