1) Simply put, I started work on Slimy Sextet to fill time and give myself something to do while waiting for things that I need in order to work on Mahou Mating, so I always have something to be working on and I never waste time waiting around for something else to get done before being able to proceed. When it's not possible for me to work on Mahou Mating, I can work on Slimy Sextet. And vice versa. Having two projects in development at once is an attempt to be more productive.
2) I have planned about 3.5x the number of sex scenes from Steamy Sextet, but that's not the only improvement. Slimy Sextet will have sprites and background art for story scenes, original music rather than recycled tracks taken from previous games, and a much higher wordcount, since I'm not limiting the game's text to purely dialogue that will be voice-acted. If you think that the game as I've presented it isn't worth $14.99, then I can recommend waiting for when the game inevitably goes on sale, but that's the price that I currently think the final product will be worth.