This is not very straightforward, really. I have to make some assumptions. if they're wrong tell me.
1) You are an LCC. That means your Embraer would have 115 seats or so if an E195; a shade over 100 if you use the E190. The A318 would have 132. That would mean that the higher seat count of the A318 would start to outweigh any trip-cost advantage on a per seat basis. So, question 1 - can you fill the additional seats?
2) Do you train in house? If the answer is yes, and you already have A320s, you already have an A320 sim. Your A320 sim works for the A318. If you already have an A320 sim and take Embraers, you will have to buy an Embraer sim. You will also need an Embraer chief captain and Embraer flight and maintenance crew structure spearate from that of your A320 fleet structure. That will cost you quite a lot of bucks.
3) The Embraer will likely burn less fuel per trip than the A318. When you factor in the amount of seats, it's probably close to parity. The airframe maintenance costs of the Embraer will likely be better than the A318. The line maintenance costs will be pretty similar (they both have a fault detection system though the A318s is a nose better). The A318s engines will be cheaper to maintain - they are bigger engines but massively derated, so won't tend to come off wing for a very very long time. The Embraers' are operating at the upper threshold of their tolerances so will tend to get pulled for EGT margin problems more quickly - expensive! Believe it or not, line maintenance about equal, engines A318 better, airframe Embraer better: net result, all square per trip. Factor in the seats. A318 advantage.
4) Price. Embraer undoubtedly cheaper to buy. Will the A318 have an advantage, overall, per seat because of the fuel and maintenance advantages? Depends how good a negotiator you are.
All is not so straightforward as it might first appear.
There are other bits and pieces that sway one way or the other - product support network, current dispatch reliability, ability to finance or lease, etc etc etc...
What would I buy?
Simple. I'd have an A319. Similar trip costs to the A318 but with 24 more seats.