If you want to make it less dense you would buy the smaller one and move the size up without changing the stitch count (software like Embird Basic can do this). I have been doing this with my recent Embroidery Library designs - they tend to be dense - and I really like them. I generally go 10% up and have been very successful that way - a little dense and yet still look "right".
Yup, me too...I have found that in my software I can increase most by 20% without really affecting the quality...but it does depend upon who digitized the design...always keep your original and Test...