Ok, I am not going to blast either one company here, and will try to give you the best opinion as a user of both companies: Linkvana Vs. Link Dominators!
Let's examine Linkvana in details! Their membership is a network of PR 2+ blogs which provides you with unlimited amounts of backlinks! With linkvana, you can start seeing very quick results for low competition keywords. Their membership fee is $150 per month and that is a fee that is affordable because it gives you guaranteed links.
Downside is that it takes a lot of effort for you to build those links! Depend how fast you type it can take a person 10 min-20 to create one link, but their system is very easy to use. You can also outsource that task to their staff, but it cost $2 per post which is a lot!
Now let's look at Link Dominators, they provide you with service. So you do not post your links and spend hours doing it, but they do it for you! Compared to Linkvana, with them you have a limit on the URL's you submit, but per each page they will give you 60+ links. At least 80% of those links are coming from very HIGH PR sites such as PR 4-10! The link dominators cost is around $10 per URL so for $100 you get around 600+ links! Of course they have different packages you can choose from.
So considering Linkvana Vs. Link Dominators, both services are good, but if you do not have the time to work on building your links, link dominators is a better choice!