I dive a 3mm boot in 50-60F water all year round. My feet aren't the first thing I notice getting cold....in fact, I hardly notice my extremities at all. Once my core gets cold, the dive is nearing the end. I'm also the kind of person who gets cold fairly easily....it was 65F out last night and I was wearing a sweater and jeans while my hubby was comfortable in shorts and a t-shirt.
I can't say what will be comfortable for you, but I would imagine in 68F water, a 3mm would be plenty....5mm seems like overkill. I second getting a boot with a good sole....and also watch out for those pesky seams running down the front of the boot (I had serious issues with the fin rubbing on the seam, and the seam rubbing on my foot, leaving me with a gnarly scab, and now a scar!).
I know you asked specifically about boots, and not gloves....but if you're considering 5mm gloves, I would strongly recommend AGAINST that. I bought a pair of 5mm gloves for lobster diving, attempted diving with them once, quickly realized that my dexterity was minimized to almost nothing (couldn't get my second glove on....had no chance in the world of operating a bolt snap). Needless to say, I don't use those gloves anymore, and now use a 3mm pair.