The 38G has been my primary ride for awhile now, before that it was the PJ (38J). I don't think the G retains E any better than the later model rides, if anything I think the J/L perform better in the vertical than the G. The G probably is a little more nimble as long as you have alt to trade and can keep the fight going downhill (it's lighter). The J adds more HP at military settings and gives you the option to engage WEP, of course the weight is higher as well.
It's been awhile but if memory serves in duels with the experienced J pilot I found that I had to really manage my energy carefully, otherwise sooner or later they would gain the top position. This also became more and more likely the longer the fight went on.
The PJ also has more internal fuel, 75% fuel in the PJ is more gallons of fuel than 75% in the G. When I was flying the J I used to take off with 75% and no drop tanks, by the time I got where I was going I was usually just about down to my fighting weight. The extra HP really helped if I had to fight before I was ready, anything more than 50% internal fuel in the G and I'm really missing my WEP.
For most my flights in the MA with the 38G I will take off with 50% and DT (actually it's usually 2 DT just because I think they look cooler with 2 loaded than one

).
Don't get my wrong, I love my poor 'ol 38G and I'm going to keep flying it, but the J is a more capable performer IMHO.