The E92 Today
Comparison
While the N55 does make marginally more power than the N54 in stock form, mod-for-mod an N54 will outperform an N55 and is, therefore, the more popular model chosen by tuners. Both engines have their benefits and drawbacks so it will hinge upon your intentions with the car. For a general rule, if you intend to modify your 335i, the N54 has better potential, but the N55 is considered more reliable for those who wish to remain stock. Again, the N54 is or can be incredibly reliable in its stock form as long as all maintenance has been meticulously performed, so I definitely would not tell anyone to exclude that engine from their search if modification is low on their list. Similarly, the N55 does have excellent potential for upgraded performance and can provide a superb platform for anyone interested in upping the horsepower.
Purchasing
If you have completed your test drive and everything is in acceptable condition, then hopefully you can agree on a price with the owner and take home a new-to-you E92 335i or 335is with the confidence that you purchased the best one for the money. Any N54 or N55 driver will tell you the potential of the cars in which you find those engines can be one of the most rewarding to drive, tune, and live with on a daily basis