To be honest, I think that decision should wait until you've finished your CPL.
Plan to have the funding for the more expensive of the two (Multi-IR) in place, but be prepared to instruct and, more importantly, to survive on an instructors wage for a number of months.
When the time comes, you'll need to assess the job market. If you see thar airlines are taking more and more modular (I'm assuming you're modular) 250-hour guys, go hell for leather into the multi-IR before the mood changes. If the market is pretty much as it is now, where you have a decent chance with 800 hours or so, get your instructor rating and start building your hours before committing to the next major expense.
That's my take on it anyhow.