Depending on the flexibility of your employer plan now and in the future, the following options may be available to you every year:
1) Traditional 401(k) contribution (pre-tax)
2) Roth 401(k) contribution (after tax)
3) Conversion of traditional 401(k) balance to Roth 401(k) (taxable)
4) Traditional IRA contribution (pre-tax)
5) Non-deductible IRA contribution (after tax)
6) Roth IRA contribution (after tax)
7) Conversion of non-deductible IRA balance to Roth IRA (taxable)
Note that only two of these options (4 and 6) phase out with higher income, and there is a "back-door Roth IRA" strategy (essentially option 5 followed by option 7) to get around the limitation on 6. So, as long as your employer's retirement plan allows options 2 and 3 above, you don't have to worry about being prohibited from Roth contributions when you want to make them. Instead, you can select traditional or Roth contributions each year depending on your current tax bracket and expected future tax brackets. Even if your company match or profit sharing must be pre-tax, your retirement plan may allow you to convert it to Roth at your discretion.
EDIT: Just re-read your post and realized you will be working for two different hospitals. In that case, it may make sense to make traditional pre-tax 401(k) contributions now to maximize your deduction. When you leave the first hospital, you can roll the 401(k) balance into an IRA (because you may not be eligible for the second hospital's retirement plan right away). Then, in the year when you have a low income you can convert your pre-tax IRA to a Roth IRA and realize the income in a lower tax bracket. Just be sure to save enough money this year to pay your additional taxes next year.
A minor point is that 401(k) accounts are considered to have better protection from creditors than IRA accounts. This can be an important consideration for doctors and may be more important to you than the small tax savings realized from timing your conversion in a low-tax year. In that case, you may just want to make Roth 401(k) contributions now and pay the tax, then roll that money directly into the second hospital's 401(k) when you become eligible. You can't roll money from an IRA to a 401(k), so once it leaves the 401(k) shelter you can never get it back in.