I say maybe, but there a number of factors to consider.
1. Is the .com already developed? If developed is it a direct competitor. If not direct, then no problem. If not developed, would it hurt if a competitor purchased it later? If yes, find another name. If no, proceed to develop.
2. Does the name fit .net well. Some names like networking, forums, hosting etc., actually sound better with .net.
3. How will you promote the site? If by print, radio, or television, you could lose significant type-ins to the .com. If all your advertising is via links, banners, and email, then it's likely all customers will get there by links and it won't matter.
4. Is it trademarkable? If yes, you may be able to TM the domain and protect it later from direct competition (but not likely use for another purpose). The current owner would probably be exempt if they owned it before you, but you may have a chance in enforcing on a subsequent domain owner. Probably not through UDRP/WIPO, due to the strict rules, but maybe by lawsuit. However, it may be less costly to get now than legal cost later.
5. Will you ultimately have to have the .com? If yes, pay the money now. Once you develop and have traffic and they prove any type-in bleed off, you'll pay much more for it later when they know you and your site.