First question is whether ...

First question is whether RO, by itself, will be able to meet the boiler feedwater quality needed.  If it can, then comes the financial question- maximize/minimize capital expenditures or life cycle cost?  RO, by itself, will almost always be less expensive than a capital installed IX system especially when considering on site regeneration system.  If the makeup water is of low TDS, IX system may be cheaper to operate than the RO (not sure where the breakeven point is but likely some where less than 200 mg/l TDS).

Many operations do no want to deal with the bulk chemical issues that are inherent to IX operations and will often select RO only (or RO with off site IX regeneration) even though there may be a higher operating cost.

The engineer inside of me says the correct answer is the lowest life cycle cost but, after doing this for 36+ years, that question is almost never asked/considered.  The direction has almost always been to provide the lowest total installed cost that will work which can easily lead towards higher operating costs and, potentially, higher life cycle costs.