Quick question on which Amazon instance type to use

Firstly, great job on ElasticSearch.
I've been reading about it all week and it seems amazing.

2 choices of instances on Amazon are:

Extra Large Instance

15 GB memory
8 EC2 Compute Units (4 virtual cores with 2 EC2 Compute Units each)
1,690 GB instance storage
64-bit platform
I/O Performance: High
EBS-Optimized Available: 1000 Mbps
API name: m1.xlarge

High-Memory Double Extra Large Instance

34.2 GB of memory
13 EC2 Compute Units (4 virtual cores with 3.25 EC2 Compute Units each)
850 GB of instance storage
64-bit platform
I/O Performance: High
EBS-Optimized Available: No
API name: m2.2xlarge

The cost ratio is around 2 High-Memory for 3 Extra-Large.

So which one is most indicated for ElasticSearch cluster overall (A cluster
of 2 HighMemory or one of 3 Extra Large)?

I know this might be a hard question to answer without knowing the use of
it, but I’m just trying to get a sense of ES resource requirements. I’d use
to index around 1 billion documents.

Thanks