Its asking a lot, but I think its worth it to have both types. For the inline, get the dual sensor outfitted for both supply and output sides of the ro membrane (not the sediment/carbon filters), this way you can monitor the performance of your membrane, see if flushing it renews it sufficiently, or if it needs to be replaced.
The dip type is good especially if it combines a pH meter, but it allows you to check tap water independently, as well as to insure that nasties have not grown in your drinking water ro pressurized storage tank. If you haven't already, you will want to get the drinking water kit to go w/ your ro system - if your fish deserve clean water, you and your family certainly do as well.
If you can only get one meter right now, I say inline dual. The more convenient to use it, the more often you will test.
Hanna Instruments is one of the biggest names in tds meters, buty they are usually more expensive than comparable models. Still, if you can find a low-priced unit online somewhere for not much more than the econo models, I would go that route instead. However, I don't think Hanna makes an inline model, there's not too many different inlines available and they aren't really branded - go straight for cost or warranty for those.