Not sure what you mean. You mention conversion, which is is taking pre-tax money from your 401k, putting it into a Roth and then paying the tax on the converted amount. Then you mention pure Roth which i do not know what that is. Most people will probably not convert money from their 401k into a Roth and will just make weekly after-tax contributions to the Roth.
If you are over 50 and start your own Roth outside of FDX you can only put in $7500/yr. The FDX 401k Roth will let you put in something like $30k including the catch up. $22,500 contribution + $7500 catch up. The only other difference I can see between the 401k Roth and Roth would the that the 401k version is going to require distributions by age 72.
Everyone's retirement plan is different and what your income will be in retirement, but I have only seen taxes go up and not come down, so hedging your bets between taxable and untaxable income in your retirement years will be beneficial.