Multiple hdd with small capacity is better if both are at the same spec and if matched with the spec of big capacity drive.
The advantages...
1. Faster Data Access (due to separate heads)
2. RAID possibility
3. Less Heat
4. Data protection
Drawbacks:
1. Low Linear Velocity (If you don't know what is this, forget it..)
2. Space occupation
3. Not future proof. (Assume, if you bought two 120GB drives instead of 250GB before one year???)
4. Price factor
How much capacity you planned to buy now? We will decide based on that