A Chance to Race

I think the oly is a better idea.

I did a HIM that used some of the IMKY bike course 5 weeks before IMKY. It was hotter (97) and more humid the day of the half, and I think that helped me. But, it was my first HIM and first IM, so my thinking may be different.

I'd do the Oly..lose to much training time even if you do mini taper and then recovery time.

Last year I did a HIM 6 weeks out from my IM and recovery was fine, I also like to use races as hard workouts when possible so it was a win win for me.

I'd say the Oly is perfect, the HIM probably too much.
To quote Gordon Byrn of "Going Long" fame- "With a deep aerobic base, it only takes a little bit of intensity over a short period of time to get the necessary physiological changes [to Peak for an IM]. Three to seven weeks out from your A-priority race is the appropriate time to start your most intense efforts." An OLY race provides exactly that intensity at exactly that time. You can think of it as "Speedwork" for your IM. You don't need much of it, but 6 weeks out is just the time for it.

Good choice.  Why take chances?  You want to hit the IM fully rested and prepared.  If you had originally shceduled a HIM into your training it would be one thing, but an "impulse" one is not the best decision usually.