I can guarantee you that the RAM will be both cheaper, and last longer then the SSD. That, and it's not a big deal to purchase an SSD in the future should you actually need it.
Since this came from Stack Overflow, I'll assume that the data you work with is very important, and you can't sustain a big data loss. If you go the SSD route, be aware that you can easily burn it out in less then a year, and possibly lose all of your data with it.
Obviously, backing up your data every day will mitigate this risk, but what you need to ask yourself is, do you really need the speedup? Could you not just get a SSD in the future should you decide that your HDD is too slow for your purposes? And for that matter, will you actually need or benefit from the extra RAM you want to get?
I'm sure you know by now the benefits and drawbacks to more RAM or a SSD - the decision is for you to make, not this community. The decision depends on your purposes. That being said, it's not a one-way road if you choose one over the other. Computer parts are cheap, and getting cheaper every day. Keep in mind that you can always upgrade in the future.
Finally, since you're even asking this question, I'll give you my honest recommendation: don't upgrade either. If you want more RAM in the future, or you want a faster storage device in the future, buy it when you need it, not before. Not only will it be cheaper (especially not purchasing it from a computer retailer), but you might not even find you need either for a little while.