Hi
For customers oven cleaning is a more affordable (lower cost) service than carpet cleaning so there is potential for more oven cleans than carpet cleans in a given period like a week or a month. The start up costs are lower but so is the return per job.
The start up costs for carpet cleaning are higher - I'd say a lot higher. Carpet cleaning takes time to develop - its a long term business because of the skills and experience and reputation it requires to be successful. The return per job is higher but so is the start up cost.
Some cleaners like to specialise, what do you want? If you've got 18 months to plan you've got an advantage - time is on your side at the moment, have a try out for a couple of days with both types of cleaning. Depending where you are, you might find someone on the forum willing for you to tag along and help out. Are you up to cleaning - its hard manual work, tiring and repetitive.
You can develop one service and add the other on as business grows. Make sure you budget for marketing; leaflets, web site, van livery, stationery, business cards, uniform, etc. Be realistic, get some training, avoidable mistakes can be very expensive.
Cheers
Grant