Rambling and probably partly incoherent post follows ...
I am a Smith
... OK, not a sword smith, but I do know a little about what makes a decent sword since I've been looking into this very subject for quite a number of years now. A "decent" sword is the product of a number of factors all working together. Each factor has a specific importance in the eye of the prospective sword-buyer. To a relative novice that is wanting a tameshigiri blade, how easily it bends becomes a very
important factor. For a higher level iaido practitioner that needs a daily practice sword, how well it is balanced becomes a more significant factor. For an epee fencer, having a blade that will retain its original shape and not stay bent or break is of overriding importance. You see where I'm heading with this right? ...
All of the individual factors aside, a "decent" Japanese style sword will have three required characteristics. It has to be resilient enough (tough) so that it doesn't shatter with a mis-placed cut, it has to be hard enough to be able to take and hold a decent edge, and it has to be heavy enough that it will perform its role as a sword properly. All Japanese sword blades have these three factors in common, no matter what else goes into their making.
The third factor, heaviness, is what precludes the use of most exotic alloys for swords. Specially treated titanium or aircraft aluminum alloys could potentially be made hard enough to properly hold an edge, although whether their maximum hardness is enough is a point of debate. However, any of these alloys made into a sword-like shape would be too light to perform in the manner that a proper sword should. Too much effort would be required for the simple act of cutting. All of the sword techniques that we practice today were made back before exotic alloys were available (back when swords were still used).
The first and second factors, toughness and hardness, have been a perpetual tradeoff, and likely will continue to be in the foreseeable future. Toughness is what gives spring steel its spring. The problem with this is that spring steel is not very hard, and so doesn’t hold an edge well. The harder you make steel, the more brittle it becomes. This is the problem with using stainless as a sword blade. It is very easy to make stainless steel hard, but extremely
difficult to make it tough enough to withstand the stress to which a sword blade is subjected. That being said, regular old carbon steel is still the best thing for sword blades, although there are many different mixtures of carbon steel. This is where the tradeoffs begin. Heat treatment
is what decides whether a given chunk of steel will be tough or hard. Traditional Japanese swords overcome the problem by differentially heat treating their swords so that the edge is hard, and the back is soft. Another way they overcome it is by various laminating methods, which help to produce the same results. These methods are what result in the “hamon”, which is the difference in the edge and back steel structure revealed by proper polishing. Various chemicals added to the steel mixture result in different methods and temperatures for hardening. These chemicals are what cause the different designations for types of steel. Steel from one country is not inherently different than steel from another country. There are good and not-so-good producers in all countries, and how well they countrol the percentages of additives in their steel is what decides whether it is good or not, not the country of origin.
So, all that being said, here’s my summation … 1) Steel is still the only decent material for sword blades. 2) The type of steel used is much less important than the person making and heat treating the blade. 3) The country of origin is much less important than the quality of the particular smelter producing the steel.
If you've made it all the way through to here, congratulations! Hope it made some sense!