Thinking of building a bench power supply with RD units. Need some help deciding which to go with.

I need a basic power supply and I know a lot of people like these units and it definitely seems better than some ebay/amazon junk so I figure I'd give it a try. I don't think I'll find a use for the 50v units so basically I'm stuck between the dps3005 and the dph3205. It is currently very late as I'm typing this so I haven't done all of the research but I'm fairly certain that since both have an input voltage requirement of 6-40v a laptop power supply (20v) would work and has sufficient filtering etc. However, since the DPS unit is a buck converter(which as far as my tired brain understands can only step voltage down), would I only be able to get a maximum of 20v DC(or some percentage of that due to loss) out of it or would I get the full 30v range? Should I consider purchasing a 40v (or close, open to suggestions) power supply like it suggests or should I stick to the laptop supply?