Dex benefits all melee. It is still the acc and eva modifier. So, you would should see a +2 acc and +2 eva if memory serves me correctly.
The level of the mob you are fighting has a logarithmic effect on stats. The bar for that change is different, and very complex, and considering most of the mobs you fight at end-game are at the top of the bar, you pretty much get negligible difference unless you do some serious min/maxing (which I find greatly disappointing). Basically, at +10 levels, there is almost no difference in those items, as the mod range is so wide, you would need to pump either Crit stat by 100+ before you would get a significant difference. And the 5 acc/3 dex/5 att alone are not going to give you any noticeable benefit. 10 att *might* be noticeable... maybe... in a parser... (for those stats I have found that 20+ is when I start to notice their impact)
So, the real question would be which one best compliments your over-all gear. The Dex is nice, but only if you are putting something into it. Since Drg is actually setup to get a weaponskill on miss, acc is not as pressing (though missing does still break skill chains). And the eva boost could be nice... but again, only if it is complementing something.
Just straight comparison between the two, the Flame Seargent's Boots are the better, but, will you actually see any benefit to them? Not alone.
As a comparison, consider what Blms do stat-wise to prepare for Garuda... They aim for something like 360+ int, and 480+ MACC if I remember correctly.
That's my opinion though:P. Take it with a grain of salt (I am kind of bitter about the min/maxing, and can't wait to see how they change stats in 2.0.