I'd get the potion first--the potion is guaranteed to work on your pet, but the lizard minion trap isn't guaranteed to catch the item ^_~. Plus, I'm sure your pet would appreciate being first in line :D.
Furthermore, the price of a nightmare potion might fluctuate--if it goes up a couple mill, you'll almost certainly need to make up for the price increase by offering more sP for it, but the lizard minion trap is in the millionaire center, so it's a sure thing that if you wait to buy it, it'll still be whatever price it is now.
Well, whichever you want more, as you can always get more sP again. ;)
The trap however gives you the advantage of possibly getting the items from a Lizard Minion and also earning an achievement. Eventually, over time, the items you get from the events will make up for what you paid for the trap (and spray, if you plan on getting that) if you sell them.
I would save, if I were you. Nightmare pets are worth it~
But, getting the trap or repellent would nab you some pretty good items that would sell for high prices. Maybe even enough to get you a Nightmare Potion. =]
Sooo...get the trap and repellent.
I would keep saving first. the lizard minion events are too random--you might buy the trap and stuff and be set back 5m, and then end up getting to enough sP for the potion without getting another event at all.
so yeah. keep saving.
[flower=Emily]