As you said... "Go with the biggest tank you can afford!"
But honestly... I have a 46g and originally thought about going the 28g nano route because it was sooooo convenient - it's just "plug and play"... right?
If you think you're going to go the coral route, which it sounds like you will, then most of those "kits" or "plug and play" setups will kinda work, but you'll probably find yourself a year down the road kicking yourself and upgrading to a larger tank anyway.
I went with a 46g because it fit the space available in my house. When I set it up, I thought it was huge. I now wish I had more room! I am SO glad I didn't go the nano route as I would've wasted a LOT of money with a setup that I would've outgrown in short order. So I'd obviously go for the 40g setup.
Regarding lighting... the wpg
rule has its pitfalls, but for initial thinking, if you get a fixture in the 4-5 watts/gallon range you can have most of the LPS
corals out there, and just a few of the SPS
corals. I have 196W of compact fluorescent over my 46g and while I'm limited in what I can keep, I've found no problem filling it with beautiful corals that seem to be happy.
You don't "need" a sump on anything. More water volume is good, but if you don't want to deal with it, don't. I don't have a sump on mine and have just decided to maintain a pretty consistent/frequent maintenance schedule to make up for the lack of one.
I think if you enjoy a planted tank, you'll enjoy a reef.
Welcome to the dark side...