"what would be the difference between the two coils...more taste? More vapor? Better throat hit?"
You really can't talk about ohms (the resistance of your atomizer 1.8 or 2.5) without also addressing what battery voltage they're being used with. For instance, a 1.8 ohm atty used at 3.3 volts should be very similar to a 2.5 atty used at 3.7 volts or a bit higher. So, not knowing what voltage you're using, hypothetically, the 1.8 should be a slightly warmer vape with a stronger TH and a bit more vapor if you had a way to test that exactly especially if you're at 3.7 voltage or above. I don't know if you're using variable voltage and changing it. One thing you should know though is that regardless of voltage, a 1.8 ohm atty will drain your battery a bit faster than 2.5 ohms will. Personally, I prefer 1.8 ohms at any voltage below 3.7; above, I prefer 2.2 ohms or 2.5 ohms. At 5 volts, I would prefer even higher ohms. I'm not a subohmer and I don't even "get" them.
I know this is probably more than you wanted to know but its the only way I know to explain. Hope it helps a bit.
Edit: I had to answer the phone mid-posting. There was only one reply when I started. Sorry if I've repeated anything already given. I'm just leavin it without reading.