There are so many factors at play, such as age, condition of each bike, size etc. Assuming both are a similar age and both are in good condition, the Raleigh will be streets ahead of the Silverback in terms of quality. The frame of the Raleigh is much better quality and is much lighter, also Ultegra is higher up the food chain than 105. At R4 500, the Raleigh is a good price. Mostly, these go for between R6-R7k on the used market. If the Raleigh is in a good condition, you will be getting a lot of bike for R4 500. Perhaps you can show links to photos of the bikes. It will give us a better idea.