If you are single and might someday be married with kids and a mortgage you could be in a lot lower tax bracket then. Single people can get to the higher tax brackets pretty quickly.

I would stick with the deductible retirement accounts until you build up a pretty good start on your nest egg. Just like the kids at the mythical Lake Woebegone that are all above average, many people worry about being so wealthy in retirement that they have will be in a high tax bracket but realistically most people will not be wealthy when they are retired which would make putting $7,500 in a deductible 401K preferable to $5,000 in a Roth after paying $2,500 in taxes.

In the long term the right answer to the question “Roth vs a Deductible IRA” question is probably to do a mixture of them so that you can minimize your taxes in retirement depending on the details

Until you get a year of two of you income saved up in your retirement accounts sticking with one of the low cost targeted retirement funds(like 2050) will be just fine and give you good diversification. After you have more money saved up you can worry about fine tuning your asset allocation