I'm new to the forum as well, but I think if you gave the numbers like pavilion, crown angle, depth, table, etc. You will get more detailed advice from some of the veterans around here.
For my thoughts, I am more sensitive to color, and I'd try going with the highest color possible as long as your SI2 is "eye clean." Spend time looking the SI2 over without a loop first to see if you see any inclusions. After looking at it for a while, use a loop to see if you can spot the inclusions. Then, look back at the stone to see if you can see the inclusions that you found in the loop. If you're happy with the SI2 grade, you may want to choose it. It will most likely be cheaper.
I'm only basing this decision on personal preferences. You may have other priorities. Also, once you get the numbers posted, others can advise you on the cut of both diamonds, which is what I would focus on between two stones like this if the price is similar.
