Allegiant is not a cattle call like Southwest. You can choose to pay $10 per person per flight ($20 round trip per person) to select seats when you book. DH and I never do but we don't take the kids with us to Vegas, and don't care if we sit apart. Even so, on both trips we were able to sit together no problem, we were assigned seats when we checked in at the airport and the flights were completely full.
be warned that nothing is free on the flight, a can of Pepsi is $2, pre-packaged snacks were $2-$4 each.
otherwise, we like Allegiant, flights have been on time and the non-stop aspect of it is great! I always plan on 2 flights to get anywhere, it's a treat to not have a layover. I wish they flew everyday instead of 4 days a week, and would love it if they could expand to offer more than one flight a day, but that's not their business plan right now. everything is a 1 shot out and back from either Sanford or Vegas, so no crew layover costs to pay for.
If we were planning on renting a car anyway, I wouldn't hesitate to use Allegiant into Sanford. We paid a little more for flights into MCO, but have better arrival times and are not getting a car, just using ME, so cost is a wash.