I think part of the "stuff more into the 401k or get an IRA" decision also needs to be based on the available investments in the 401k. Not all employers have that good a mix in there if you want to diversify your investments, so not everything is only dictated by the fees. They are, however, a negative indicator - the higher the fees, the more you only want to put in enough to grab a potential employer match.
If you have a 401k available to you that offers a good mix of low-cost investments (which usually means that it's with one of the low cost providers like Fidelity, Vanguard or similar) then it makes sense to max out the contributions first. If the fund choice is crummy and/or it's with a high cost provider, then you get better returns over time if you only contribute enough to put the money in to get the employer match, if any, and invest the rest of the money yourself into low-cost choices. IMHO, you want to see fees on the investments that are in the 0.1% range or considerably lower, although I do invest in a couple of funds that have fees in the 0.4-0.5 range because of the lack of alternatives.
One of the big advantages of 401ks is that they are considered protected assets so if some financial disaster strikes, your creditors usually can't grab assets in a 401k. Not all other retirement saving choices are protected like that, for Roth IRAs at least it varies from state to state. For me, the protection however doesn't outweigh the financial damage you can do to yourself by investing into a 401k with high fees and crummy choices. That's a choice everybody has to make for themselves.
Next, taxes and Roths. Regular 401ks are tax deferred - put the money in before tax, pay ordinary income tax when you pull it out - which can be helpful if you can use that as part of an income tax management strategy. Contributing to a regular 401k just for the tax saving does make about as much sense to me as keeping a mortgage around for the mortgage interest deduction, you'll have to get more out of it than just the tax advantage. Some employers now also offer a Roth 401k which is fed post-tax money, but you don't have to pay taxes on withdrawals made in retirement. If your employer offers that, it may make sense to put some money into a regular 401k and some into a Roth 401k.
Roth IRAs then allow you to contribute an additional post tax $5500 ($6500 if you're over 50) to retirement savings and because they're a Roth, you don't have to pay taxes on the earnings when you take them out in retirement. As ProDarwin already mentioned, you can also withdraw your contributions without taxes or penalties if you have to, as long as you leave the earnings in. Some people - like myself - make use of that feature and use Roth IRAs as a combination of retirement savings and backup emergency fund. I try to keep enough money around for "regular" emergencies like failed hot water heaters, furnaces, Porsche engines eating their IMS bearings and stuff like that.
Money that goes into the "you're fired" fund ends up mostly in Roth IRAs for both my wife and myself. That's on the assumption that this money will (hopefully) stay invested a lot longer because those dire emergencies are much rarer than the usual item breakage, having to take pets to the vet etc. That money at least needs to be able to keep up with inflation, which you currently can't with a regular savings account.