In general, I would say that a duplex would be a better investment than the condo, but we are missing some important details to offer advice in this particular situation.
First, how much are the monthly association dues for the $115,000 condo you referred to? $1,600/mo rent for a $115,000 condo sounds pretty good at first blush (but I come from an area where even the 1% rule is tough to find), but after you factor in dues it might be closer to the norm for your area.
Second, how much rent do you think you could get total for the typical $300,000 duplex?
I would let the numbers for your area dictate the decision. The agent you spoke with had a good point about reserves, but if you have other means of covering a short term expense (like a credit card), then go for the best overall investment. Be sure to look at the big picture though - if the typical $300,000 duplex has an older roof and older mechanicals then you will be looking at much larger expenses than a couple oven/range replacements.