Well Flash is already a complete product - admittedly a proprietary one from Adobe.
HTML 5 is still at an early draft stage and may be significantly different by the time it finally becomes a standard. Anyway it just adds a few additional tags to what is already in the HTML 4 standard (even though 95% of people are still using HTML 3.2 that has been obsolete for over 15 years). Some of the proposed new tags for HTML 5 have already had their reason for being added disappear - eg IE6 was the last browser being used to not understand the standard object tag making the embed tag unnecessary and the pattern attribute in HTML 5 makes the required attribute that was also proposed in HTML 5 unnecessary.
Also Flash will take longer to learn as it is a new language. HTML 5 is just a few extra tags added to the current standard HTML 4.
The fancy stuff in HTML5 and CSS3 is likely to work on almost all browsers eventually, and you can provide graceful fallback options for browsers that don't support it all. Flash is unlikely to ever work on any iOS device, which renders it pretty pointless in a world where iPhones and iPads have an ever increasing market share.
Any posts I write in Arial are on my mobile phone, so please excuse typos etc. Any posts I write in Verdana are on a PC, so feel free to berate me mercilessly for any mistakes
As has been said, as a web developer, you'll not get around to learning HTML5. And it's not true that HTML5 is just a bunch of extra tags. It's a lot more than that and it will be of great importance to get to know HTML5, if you want to have a sustainable business in the web development world.
Flash still has its advantages in some areas like animation, for example, but the newer technologies are gaining ground at a mind-boggling speed, so I'd definitely opt to go with what will be a requirement in this industry unless you just do it for a hobby.
And t's not true that HTML5 is just a bunch of extra tags.
Flash for web is the past, but it's still a useful tool at least for sketching animations and interactions. And there's still countless online games that use it.
Flash software is just great for making cool animations for your web site so if you miss it then you`ll miss a lot,so go for it=> i have learned it in my web designing course and i give it a thumb up.good luck!