I think I'll go for xEnviro, to me it looks much more realistic than SMP4... I give up on SMP, after 4 versions, the clouds look so ugly and unrealistic... I'm highly doubtful that version 4.5 of SMP will offer any improvements.
xEnviro has bugs, too. The transition of weather is not smooth. It has not been updated since the past December. I advise not to buy it at least until the next update.
IMO, as a weather engine, the price of xEnviro is just ridiculous, even higher than the sim itself. I don't think xEnviro is harder to develop than X-plane. I'll definitely buy it if it costs 20$ or 30$ since there is no better choice for x-plane. Srsly, I don't know what happened to Flight sim community recently. I knew a good add-on needs a lot of work. But over 100$? (yeah I'm talking about you guys, FSLabs and PMDG). I don't think so. It really push a lot of potential customer to use them illegally.