You will find that the R will absolutely eat the l for lunch. With a few tweaks, we were extremely happy with our R5 in corn, beans and oats. We went from a very nice l2 to the R5, and it was a huge jump.
I think it depends on the crop. We had an l3 and demoed an R6 when they came out and I preferred the l in corn. The sample was cleaner. In tough corn the R6 had more capacity. In beans it was pretty close. The 6 had more capacity in good dry conditions. In green stem beans the l was better. We eventually went to a R60. It had equal to better capcity, but they are not the easiest things to keep going. We eventually went to a 62 then 72's and the 2 series blows the R6 and R60's away. The R6 has a bigger grain tank which is about the biggest advantage I see. If I were in your shoes I would go for the l series. The R6 and R60 were fine early on, but they do not age well and a machine with 2000 hours on it will give you your share of headaches.
I'd take a R6 anyday if conditions were comparable. I wouldn't take the R6 out without shimming, stacking, or installing channel helical bars. Green or large quantities of straw will not flow with standard helicals. Thats what worked the tar out of them and gave them a bad name hear and there. Check to see if it has tailings return to cylinder and a chopper. Otherwise a good look over with someone familiar would be in line but don't let someone scare you to much for any machine that old will likely need some work.