I think it is important to consider what @Tyler_Huel has said about taking the RTD together with your usual meals, and not “instead of”.
If you are looking to put on weight one think is clear: you need to consume more calories that what you are currently having.
Thus, Huel RTD could be helpful, since it has 400kcal in 500ml (which for me it is really easy to consume). However, 400kcal are not much if this product is replacing a big meal or makes you full for long.
In fact, Huel powder with extras (peanut butter, fruits, honey, syrup, coconut oil/flakes, cookies…) could be more caloric dense than Huel RTD. Furthermore, it offers you more alternatives (and easier) to increase the caloric output of your shake in order to meet you calorie goals. Nevertheless, if you have had trouble with protein powders, you could find Huel powder to be equally (if not more) filling.
Thus, I would consider both to be good options. Unfortunately, there are no smaller samples for the powder or starter kits where you can try different products; thus you will have to make a “considerable” economic commitment to try both.