First off, I am not wildly convinced about upper chambers in general. It seems to me that a lot of countries with unicameral systems - the Scandinavians, most of the Balkans, the longer-lasting democracies of Africa and Central America - do perfectly well without. Croatia abolished its upper house a few years back, as part of a general liberalisation of its political system. The Senate Chamber of the Hungarian parliament is beautiful but empty, and there is no cry to fill it up again.
Conversely I am not persuaded that any of the countries with two fully elected chambers really benefits much from the experience. Certainly there is no good argument to be had about democratic legitimacy; if you have one house whose members are entirely elected on a fair voting system, why do you need two? Both Romania and Italy have experienced constitutional crises in the last couple of years precisely because the will of the voters was reflected slightly differently in the two houses. It seems to me an avoidable error.
If you have a fully federal system, as in Germany or Switzerland, it is of course a different matter; when your country is at least in theory composed of different units, they deserve formal representation of some kind in the legislature. I have to say that the German solution, of each of the Länder governments appointing its own Bundesrat delegates, strikes me as preferable, but there are many other ways of doing it. And there are also unicameral countries of considerable internal diversity.
The arguments in favour of having an upper house all revolve around checking on the executive rushing through bad legislation. But I think this needs some unpacking. The issue of "rushing" can be dealt with by having a properly elected legislative chamber which actually scrutinises the executive. (I have written before of the foolishness of restricting the membership of the executive to parliamentarians.) The issue of quality control is trickier. Testing for legality and constitutionality is essential but also fairly straightforward, and should be done by a properly constituted and empowered supreme court (and the UK has just taken a step in that direction). But I am uncomfortable with the idea that an upper chamber should be able to vote down a government proposal which has been passed in a democratically elected lower house because they disagree with it politically.
It does seem to me reasonable that there could be a revising process where the proposals of the lower house go before a panel of experts, preferably selected by an independent panel for a reasonably long term of office and barred from running for election to the lower house themselves while so serving. You could build regional (and gender and ethnic) representation into such a system rather easily. In fact that is not far off what the House of Lords is at the moment, once you get rid of felons, the remaining hereditaries, the bishops, the completely senile and those who are just there because they are rich.
But an elected upper chamber will simply have exactly the same kinds of people in it as the elected lower chamber, with the important difference (on current British propsals) that it will at least in part be elected fairly. It would be better to get rid of it entirely. I don't see what value that adds to the legislative process, and I do see it as a distraction from the key issue of a democratically elected House of Commons.