That is a very hard question to answer because that will be based on your business requirements.
What I can say is that
1) Like almost all other extension provides we do not develop any further on J2.5. J2.5 has long been 'dead' from Joomla's point of view and most extension developers have stopped J2.5 development / big fixes. We are currently fielding questions related to J2.5 but that will also stop in the future - sorry I cannot give you an exact date yet.
2) When we develop code for Joomdle we will leverage the features of Joomla, its APIs, libraries, etc which may or may not be supported in J2.5.
3) Most extensions build on J2.5 will run happily in J3, therefore you may wish test your custom extensions in J3 - perhaps they work?
4) At some point you will likely need to migrate to J3 and thus your purchase / customization will need to happen - it is just a matter of when. We do not know the length of time your site will need to live or how much investment you are prepared to take - this are your business needs.
5) You will not get a lot of support from 3rd party vendors, thus you will need to have all development knowledge in house or contracted. This of course is a cost to you but again it is your business issues not technical.
Personally, I would ask myself:
- Do I have the skills to manage the J2.5 extensions and Joomla myself (opportunity cost of your time doing something else) or with help from contractors (costs money) vs support from extension developers
- Will changes need to occur in the future? If so, it is likely to cost more money again because of older code
- Can I live without security updates and the like because of older unsupported code.
- Will I likely need additional extensions in the future? A new cool module, etc. It is likely to be only available for J3. Thus at some point will there be an extension that tips you in favor of J3.
I know my answers are generic and vague - it is very difficult to answer your question as it is not specific to particular features, etc.
For myself, ie my real business, I too am presented with similar problems as yourself and am currently running a mixed J3 and J2.5 environment. I have been buying time with my J2.5 but I am constantly worry about security issues and wishing I was already on J3 to take advantage of many new extensions. My problem is that I have customisation that I either throw away and change my business or get re-developed. Both of which are less than desirable. But I know I must move forward to meet my customers lifestyle which J2.5 does not provide as easily and after long hard deliberations, the stress of staying on J2.5 is greater than the cost of moving. Your situation could be very different.
