the only reason to go R99 is to make it PVP or to make it +6 as well, if not past +4 it will have less pdef then a +6 Seraph..
also a +6 Bless Seraph set has more pdef and other minor boosts then a +6 Standard R99 set
add in the fact the set bonus of R99 hvy is not quite that exciting.. only reason to consider it for me personally is the PVP aspect but as you need 50+ PVP stones for that which will be very rare aka expensive to get this is not realistic for the average player to get..
I actually had a +6 blessed immo set (still have, but use on my iss). I knew it was absolutely better than a normal Twilight set, but it was still very early in GoD, so not a lot of people had much solid info on exact stats. I was a bit saddened to find that my stats were still better in that than the full Seraph set I worked hard to put together. I had to +4/5 a few pieces on it just to make the pdef slightly better than the Immo set.
So if talking overall stats vs cost for PvE alone, the seraph at +6 and a few blessed r 95 scrolls as you can find/afford would absolutely be the way to go.
If going for high end PvP/Oly, then you are pretty much relegated to working on an eternal set for the PvP options alone. (despite how long it may actually take to get them applied)
Of course, but would be cheaper by several bullion than trying to do the same with r99.
And if you already have the +6 seraph, you're already set vs an average eternal set for PvE and most PvP even, so it's not like you would need to rush to get them. Just something to work towards or get lucky with during an event.