If you had to pick one between Rogue Legacy and Spelunky which would it be and why?
It's about what I prefer at a given moment.
Spelunky is a game all about learning and mastering skills, where the sense of progression is based upon your ability to react to the same circumstances altered for each attempt and responding better. Where the ultimate reward is completion from your own skills of mastering the challenge.
When you die, you start all over and the only thing you have to hold to really is what can you recall previously. I seek this out when I wish to enjoy a struggle, a challenge, and have that sense of accomplishment for how far I get compared to times before. Getting as close to or passing the finish line in one sprint because that's all I was going for.
Rogue Legacy is an action game where if you let it, most every attempt will usher a form of progression and sense of completion. You beat a boss and it remains defeated, you got enough gold to buy a stat increase it will carry over for all characters no matter how many times the world changes, you discover something like armour blueprint you will have it ready to buy and use forever, it is not lost. Learning how to best react to situations and enemies, knowing how everything behaviours will get you farther than not learning, but for some they can jut "grind" to eventually overcome most of these issues and going straight to a boss you haven't defeated yet.
You can play it without buying upgrades of stats save for perhaps a few, and it does reward skill of mastery with it's controls and techniques. I've got as much out of it by ignoring all but the added abilities and that found in the castle itself with no grinding what so ever, as I did by trying to get all the stats and builds possible. No matter what however, that sense of beating boss by boss and the base design of what you play is always present. Thus this is a game I play to feel like I've accomplished something, that I got something out of my activities more often than not and my profile is better off it.
Both great games, both amazingly fun times if you can get into them (and plenty of reasons not to for either), but if I had to choose I would go for Rogue Legacy just because I can enjoy myself more with the variety of different builds of abilities or stats focuses between profiles, on top of improving in skill. It offers me more options than Spelunky's style of one base character type where I may play better or worse than before, ultimately it's the same type of play each time.