AMD K6-266

Cost

If you're thinking about cost, the Pentium II - 266 is about $20 - $50 more than a K6/266, as far as motherboards are concerned expect to pay at least $30 more for a Slot-1 LX motherboard when compared to an average priced Socket-7 AGP board.  The final difference in price you'll see when buying a Pentium II - 266 over a K6/266 will probably be, at most, $100...and depending on how much you shop around, that difference could be taken down to as low as $60.  If you already have a motherboard that does support the K6/266 completely, then the obvious choice is a K6/266.  However if you either don't have a K6/266 compliant motherboard, or are making a fresh upgrade the choice becomes a bit more difficult.

Overclocking

With the price factors virtually eliminated, we must now consider reliability/stability of the K6/266 in comparison to the Pentium II - 266.  You can almost bet on buying a clock-locked Pentium II - 266 CPU now, with the highest achievable clock multiplier setting on the processor being 4.0x.  AMD's K6/266 on the other hand has yet to be clock-locked, with the highest reported clock multiplier ever used was 5.5x.  However a clock multiplier means nothing if you cannot put it to good use.  For example, chances are that you won't be able to clock a K6/266 at 66MHz x 5.5 too easily.

We have already established that the K6/266 can make it up to 333/338MHz fairly easily on most motherboards, and provided you have decent cooling (boxed heatsink/fan would do just fine) the Pentium II - 266 can make it up to 338MHz without any problems.  With the Pentium II you will probably have an easier time since the processor's voltage settings aren't controlled by the user, and don't need to be adjusted for overclocking purposes.  The K6/266 may give you a few problems here and there with overclocking, but once you achieve that point of stability the processor should be fine.

100MHz Bus

The Pentium II - 266 and the K6/266 are both able to run at the 100MHz bus speed, albeit an unofficially supported "feature" of the two processors.  The Pentium II - 266 can be used in conjunction with the ~92MHz Bus Speed setting on the ABIT LX6 to provide you with stellar performance, unfortunately the LX6 doesn't provide the user with an option for a 1/3 PCI Clock Divider, meaning your PCI Bus is put under tremendous stress as you increase your Memory Bus Speed.  Newer Socket-7 and the upcoming Super7 boards on the other hand do support an asynchronous PCI/AGP Bus Clock (~32/64MHz respectively), making the 100MHz bus speed much easier to use with one of these motherboards. 

It has been proven that Socket-7 systems receive a much larger performance boost from the 100MHz bus speed when compared to Slot-1 systems, simply because the L2 cache of Socket-7 systems receives a fairly large performance boost as a result of the 100MHz operating frequency.  Slot-1 systems don't derive the L2 cache speed from the system bus speed, rather the processor's internal clock, making the 100MHz bus irrelevant to L2 cache speed in Pentium II systems.

Can the K6/266 operate at a 100MHz bus frequency?  Short answer, yes, it can, but can it in all cases?  Nope.  The K6/266 wasn't designed for use at the 100MHz bus frequency, although it does work at the bus speed much more reliably than any other Socket-7 processor.  Using the MTech R581-A, the K6/266 managed to boot at 100MHz x 2.5 and 100MHz x 3.0, unfortunately Windows 95 would not operate with enough stability in order to consider it a reliable setting.  There have been success stories across the net with the K6/266 running at the 100MHz bus speed, as well as benchmarks, which prove that the K6/266 can operate at the 100MHz bus speed, however you must remember that not all K6/266's are created equal...neither are all motherboards.   What does this mean?  Basically, it's a risk, don't expect the K6/266 to work flawlessly at the 100MHz bus speed, then again don't expect it to crap out at 100MHz.   At the 90MHz setting on the MTech board Windows was much more stable, unfortunately the test system was plagued by Windows 95 registry corruption during normal operation.   Again, there have been success stories with the K6/266 and the R581-A at both the 100MHz & 90MHz bus speed settings, and in the tests conducted here the K6/266 did manage to run Winstone at least once however for the most part the setting was unusable for even light operation (basic Windows tasks, copying, deleting, etc...). 

Intel or AMD?

As mentioned above, if you currently have a motherboard that does support the K6/266, then the choice is clear, go with the AMD chip.   In the tests Anand Tech performed, the K6/266 came in about 6 - 7% slower than an equivalently configured Pentium II - 266 LX system in Business Winstone tests, and about 10 - 12% slower than the Pentium II in High End Winstone tests.  At 300MHz (overclocked) the K6 still fell behind the Pentium II in most tests, however it did manage to cut the gap by a few percentage points. 

If you want the best possible Quake 2 performance, the K6/266 still doesn't reach Pentium II performance levels, even with a Voodoo², so if you're a Quake 2 fanatic then the Pentium II - 266 should be your choice.  Of course, if you're a die hard AMD fan (or prefer not to buy Intel processors) then the K6/266 is an ideal choice, in most cases however, the Pentium II - 266 is a better buy.  Provided you buy a current revision of the Pentium II - 266, chances are you'll able to clock it at 100 x 3.0 on a BX Board come April 15 of this year.   Considering that the Pentium II - 266 is a chip that's almost begging to be overclocked with a higher bus speed, and the K6/266 isn't as overclockable (in terms of the percentage of success rates) the Pentium II - 266 is a better choice in this case.

Conclusion

The best way to phrase the overall quality K6/266 is this: A processor that would've been great had it been released 5 months ago, unfortunately the performance and physical enhancements are just too little too late on AMD's part.  Hopefully the K6-3D won't fit this mold quite as well...we can only wait and see.