Sabrina it very much depends.
In general if you rent land up to say around half of one sim (32,000) you would expect that the rental cost in either US dollars or Lindens to be less than the overall cost of premium membership and direct Linden Labs tier cost. After all no body would rent land at all if it cost more money to rent privately, than from Linden Labs
Having said that my personal view would be to rent land from a private Island estate rather than on the mainland. The reason being you may be better protected against griefers as most estate owners take their business seriously and value a continual income stream from happy tenants.
Anshe Chung is the largest private estate owner in Second Life and I would endorse her business having been a previous client of hers. Also Desmond Shang of Caledon (my current location) is a very reasonable and fair person.
Most private Islands also offer the option of buying land, however any contract is between them and you, rather than Linden.
Finally on the private Island estates I have found sim performance to be better than mainland property, mainly because Estate owners can veto certain types of scripts and other activities that cause undue lag, one example being camping chairs and casinos.
With mainland property you are renting directly from Linden. On that point land can be easier to dispose of when any project has finished as you can sell it on, not all estate owners offer unrestricted sale options. One sim plus would usually be more cost effective to purchase via Linden, however you will possibly have to buy a sim at auction if you want all your land in one place, average complete sim costs are around $1,000 US and tier costs of $195.00 per month
Of course the last option is for you to purchase a private estate yourself. Full details are on this site but a single private Island sim would cost about $1,200 to purchase and the tier cost is premium basic membership ($22.50 per quarter) and $195.00 per month. Private Estate owners have more control over their virtual environment including such things as water level, greater terro form limits,, and day/night control options
I suggest you surf over to
www.anshechung.com and review the various price options, comparing them with the Linden rates as a base comparison.
Finally I strongly advise you to lurk for a time in any area you are thinking of buying to make a judgement, in other words a bit like a house survey in first life. And visit some private estates and mainland areas to compare.
Hope this helps
Regards
John