Well I don't have the 5.1 or 1.5, but I did have the Bravo-1 for about a year and used it several times before selling it, I also currently have the 4.1 in M390.
I think both handles are quite comfortable, although the 4.1 is noticeably more grippy, you'll also get an upgrade in steel going from A2 to 3V (better edge holding and toughness, though it will be a little more time consuming to sharpen). As far as thickness, I don't think .25" thick blades are more of a hindrance than a help in medium sized or smaller woods blades. If it were me I'd probably buy the 5.1 and the once you get it in person and can try out the ergonomics sell one or the other (or keep both) If I could only choose one I'd probably go 5.1. Don't worry about the sharpening and loosing steel too much as long as you don't reprofile after every use it should last you decades for either knife, the only knives I've seen where the wear was noticeable was in a Spyderco Military that had been used hard 5 days a week for 3 years straight (like hair splitting sharp to spoon dull every day and therefor a reprofiling every day) of course that's a lot smaller knife, both thinner and less edge to spread the cutting out on, and you probably won't use yours on concrete bags, drywall, etc daily.
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