I will advise you to take the Srinagar Leh route. It is better, shorter and there is plenty of tourist infrastructure on this route. If you start from Srinagar in morning you should be able to reach Kargil by dusk. From Kargil one can start next day and reach Leh by evening. But by the manali route it will take you any where between 4-6 days to reach leh, depending on the condition of the road and weather. This road is frequently closed due to snow and landslides and hence you can get stranded here for few days at a time. Apart from this the manali route takes you through the Lahoul Spiti valley which is at around 13,000+ feet. So the views are splendid but the route is tough.