I had both the DF500 and the DF700XL, I also had all 3 accompanying systainers which gave me a selection of bits from 4mm right through to 14mm.
When I got the DF500 I thought it was the bee's knees and it worked very well, when I got the DF700XL although it is heavier I found the ergonomics in use to be much better, it is better balanced. This has to be taken that I am only a hobbyist and not doing thousands of holes a week where the additional weight of the DF700XL may then start to be an issue.
When I got the Seneca adapter and Domishims, I did some testing with the DF700XL using all the DF500's cutters and found that I could do everything with that combination that I could with the DF500.
I kept the DF500 Domino Systainer along with 4mm, 5mm, 6mm bits and sold my DF500 along with 5mm*, 8mm and 10mm bits and used the money recovered to buy a TS55 and Rail kit.
*The DF500 comes with a 5mm bit and the DF500 Domino Systainer also has a 5mm bit.
Personally for me in hindsight, I should have bought just the DF700XL and all 3x Domino Systainers along with the Seneca adapter and Domishims (and probably now, also the Seneca DF700XL Height Adjuster).
It is your decision and I am sure you will get many other differing suggestions as to what to do, but for me this is the fit that works.