Dilemma: Please help me select a class
Vi Veri Veniversum Vivus Vici
In general, both classes are more complex than your warrior. Thieves require much more attention to positioning, and their fragility means any failed dodge could be fatal. Engineers require an enormous amount of button pressing, almost on a level with elementalists. Neither class will give you overall better results than a warrior, but both provide very different playstyles.
In WvW, Thieves are better for solo roaming due to stealth. Engies are better for medium sized groups or zergs because grenades have a long range and they’re awesome. Both classes have good mobility. Their playstyles are really different, so you will likely be picking on which you find fun. Give both a try for a few levels and see
Snit Dirtnap (Thief)
Ratbag Dogsticker (Guardian)
…Yak’s Bend
Ratbag Dogsticker (Guardian)
…Yak’s Bend