If I had my choice I'd scale up on those ESX hosts and add more memory rather than scaling out. Scaling out requires a lot more capital costs both in terms of servers and ESX licenses. So long as you have enough failover capacity on your hosts should one fail, I'd go with the route of adding memory rather than adding hosts.
If cost wasn't an option then the ideal for me would be more hosts, less strain on each individual host. However with your hosts having 4 quad core CPUs providing you are not running masses of vCPUs in your vm's I would go for increasing RAM in your existing hosts. As vmatty said make sure you will still leave yourself enough failover capacity, when you increase the virtual machines.
I also agree with adding RAM to the systems and might be worth looking around your environment and make sure the resources you have given out are fully being used. You might be able to finnd somme VM's that are not using all the memory you have given it and might leave you in a position to reclaim some of your resources.
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