Hello Imen,
OSPF doesn’t make a difference between inter-area or intra-area external routes. It doesn’t matter where the external route originated, it’s the cost that makes the difference.
When I booted up this lab again, I did find something interesting. When I wrote this lesson, I used IOS 12.4 and this is the list that OSPF uses for path selection:
- Intra-Area (O)
- Inter-Area (O IA)
- External Type 1 (E1)
- NSSA Type 1 (N1)
- External Type 2 (E2)
- NSSA Type 2 (N2)
Since Cisco IOS release 15.1(2)S, Cisco uses the path selection order from RFC 3101 which obsoletes RFC 1587. What this means is that it prefers N1 routes before E1 and N2 over E2 routes. In other words, the preferred path now is:
- Intra-Area (O)
- Inter-Area (O IA)
- NSSA Type 1 (N1)
- External Type 1 (E1)
- NSSA Type 2 (N2)
- External Type 2 (E2)
So if you have an N1 route and an E1 route, it doesn’t matter where those originated…OSPF will prefer the N1 route (if you run at least IOS 5.1(2)S 
Hope this helps!
Rene