There's a lot of variables that could effect your decision. Listing them all will make the decision seem really hard, but it's not. If you have very low mineral levels in your water, then it doesn't matter much which ratio you use. Pick the one that is easiest to work with, but still fits your mashtun.
The main variable: mash pH vs mineral buffering power.
Meaning, how much does your water resist the malt's natural desire to lower the pH to 5.2 to 5.4? You can go so far as to get a detailed water report and enter it all in a spreadsheet.
Or, on brewday, you can just line up a few coffee cups with an ounce of crushed malt and then add hot water to each. Start with 2 oz, then 2.5, then 3 as this will represent 1:1, 1.25:1 and 1.5:1 ratios (as qt/lb). let them rest for 15 minutes at 150-155F (microwave them for 5 seconds and stir to bring the temp up).
Then just use test strips from your LHBS on a little of the wort cooled to room temp. The one closest to 5.4 wins.
You can just toss all of the test wort into your mashtun along with the rest of the grain and water. Easy, quick and no waste!