The ms251 will save you some weight and has plenty of power to pull a 16 or 18" bar. I have cut a few trees that were in the 22-24" range with my 251 and 18" bar and a couple that were 30". But, if the majority of your wood is 18" or less all you really need is a ms170 with a 16" bar. They are only $180. Buy an extra chain and 2 in 1 file to go with it and you will be set for quite a while.
No need to buy a saw because of a warranty as someone said above. A 5 year warranty is totally useless on a chainsaw. The odds you will have a manufacturers defect after a year are slim to none. Warranty is not going to cover a worn chain, worn clutch, worn bar, clogged up carb, pull cord, fuel hose, or blown up saw if you don't change the air filter or cut with it wide open and burn it up. There aren't many occasions when a saw fails that it's the manufacturers fault. The most common failures I see are due to a lack of maintenance like running a dull chain, running chain too tight or too loose, storing saw for long periods, not replacing air filter and the main cause I see is loaning a saw to a friend (this will ruin a saw almost every time because they won't sharpen the chain, they only run it wide open and they put straight gas in it).