Without a lot more details the only real answer is "it depends".
In later versions of SQL Express I believe that the governor tweaks present in MSDE (the precursor to "express edition" back in the SQL2000 days) have now all been removed so other than the maximum cores limit there is nothing to consider here.
The only rule of thumb that we can give with this little information with regard to memory is that if your database (or databases in total if you have more then one on the same server) is considerably smaller than 1Gb then the memory limit will not affect you as SQL Server is unlikely to ever use more than the limit while running that database.
As your database size approaches the limit you need to judge what your applications normal working set is as a subset of that data footprint - you can run much larger databases than the memory limit efficiently enough if most of the data is usually not used regularly (so is not normally needed to be kept in RAM for fast access) and your database design has good index provision (so you are never table scanning large structures where an index scan would otherwise do, or index scanning where an index seek would otherwise do) and your code it well designed with respect to being able to use the indexes well (i.e. be careful with functions in JOIN and WHERE clauses that would otherwise be able to make use of index structures).
Data sizing can be a very complex beast. If your database starts small then you can probably just go with Express and monitor efficiency as the data grows. If you have speed problems while the data is still small then the problem is likely in your data structure design or code design rather than in SQL Express' artificial limits. If you have a good idea of how much data you will need to support in X months time, generate sample data of that size (and a set several times that size) and run performance benchmarks against your app so see if it copes (and if not, to see if any problem is SQL Express or a problem you can fix in your structures/code). That can be time consuming but is the only real way to get a relatively unqualified answer to this sort of question.