This might not be the most popular opinion but if there's no match, I'd reconsider participating if I were you. If you are likely to leave the country before you retire, you might be better served by something other than a 401(k).
It really comes down to paying income tax on this money now, or later.
Traditional: You deduct your contribution from your income delaying taxation.
Roth: You pay the income taxes on your contribution, shielding your distributions from income taxes.
What would be the "best" could only come from really looking at your income, the tax tables, and how many years you'll be in the country. Essentially, do you want to pay income taxes on your contributions incrementally each year, or do you want to pay all at once when you liquidate the account?
Lets say you're contributing $5,000 to your 401(k) now, and will for the next 5 years and your top tax bracket is 12% (and you earn 7% in the market annually):
You deduct $5,000 from your income, saving $600 in taxes each year. When you leave in five years you'll have an account balance of about $30,766 thanks to your gains. You'll have to liquidate your account and owe income taxes on $30,766 which might even push part of that amount in to the next tax bracket at 15%. You saved $3,000 in taxes over the 5 years, but you'll owe about $3690 + another 10% penalty for $3,076 for a total of $6,766 in tax and penalty.
If you made these contributions on a Roth basis, you contribute $5,000 and pay $600 in income tax each year. In five years you'll have the same account balance of $30,766 thanks to good fortune in the market. When you liquidate, you get to take your $25,000 of contributions back with no tax consequence. The $5,766 of gains will be subject to income taxes and penalty for a total of about $1,270. With the Roth you will have paid $4,270, $3,000 over the five years in additional income tax and $1,270 when you liquidate your account.
In this hypothetical, your total cost for the Traditional is about $6,766, versus a total cost for the Roth of $4,270. All other things being equal, your Traditional Account leaves you with about $24,000 but the Roth would leave you with about $26,496.
Now, there's an argument to be made that maybe you wouldn't contribute the full $5,000 each year if you had to pay tax on that amount, maybe you'd only contribute $4,464. But, even running those numbers your account balance after 5 years would be $27,468, you contributed $22,320 for a gain of $5,168; taxes and penalty on your gain is $1,132 plus $2,678 in taxes on your contributions over the five years for a net total of $23,657.
So, you can see these totals aren't very far apart from eachother particularly when you control for tax rate. It's really going to hinge on where your annual income falls on the tax table each year and where it would fall the year you'd be leaving. Maybe your traditional contributions spare you from creeping up in to a higher marginal rate each year and maybe if paying all the tax in the future would cause you to creep in to a higher bracket when you leave. All things being equal, this is a coin flip, you're not going to get away from the taxes though.
Obviously the math skews dramatically toward an employer plan when there's a match, a dollar for dollar match on your 401K contribution would result in a balance of over $60k versus $30k after the same 5 years. Also, even if there's no match but you're contributing above the IRA limits you might want to go for a blend. Unless you were going to switch to a Roth paradigm the benefit of payroll deductions probably outweighs switching from the 401(k) to a Traditional IRA. Be vigilant about the fees you're paying in the 401(k) fund options, but there's not a lot of wiggle room here, and not a lot of net difference.
And for whatever my personal opinion is worth, without looking at your specifics, I'd probably stop contributing to the 401(k) and start my own Roth IRA somewhere with high quality no-commission low-fee funds; Vanguard or Schwab.